Why do humans (usually) strongly prefer privacy when making love?

Probably because they are physically vulnerable at that time.  Also
another suitor is less likely to take advantage of the situation.

It's also illegal to engage in sexual acts while in public, in most
developed places worldwide.

Not all humans do require privacy. Amongst certain primitive peoples
sex is not something that is relegated to the hidden or the shameful
and thus there is not a default need for privacy.

In Western civilization the need for privacy is mostly a function of
the Puritan/Judeo-Christian idea that sex is a necessary evil that one
does not speak of.

I don't have time to look right now but I will come back with links.
In the meantime I suggest you look to sociology for your answers. The
comment above, while not invalid (there are certain camps in
evolutionary psychology that will back this up) isn't the whole of the
answer. As thinking animals the reasons for the need for privacy is
much more complex than fear of rape/competition.

Well, animals would be just as vulnerable during the act, yet they
don't seem to seek privacy to the extent humans do.

As for illegality, I strongly suspect that its illegality stems from
some more basic and ancient reason ...

As far as puritanical Judeo-Christianity is concerned, I'd be willing
to bet that adherents of other religions, like Hindus, Buddhists, etc.
feel the same way, and probably have for many centuries.

I continue to contend that a desire to be alone among amorous couples
is virtually (although not completely) universal.

The question is (as it is so often): "Why"?

Incidentally, arlenegreen-ga, I look forward to your providing some
evidence for your two unsubstantiated assertions, viz:

"Amongst certain primitive peoples
sex is not something that is relegated to the hidden or the shameful
and thus there is not a default need for privacy."

"In Western civilization the need for privacy is mostly a function of
the Puritan/Judeo-Christian idea that sex is a necessary evil that one
does not speak of."

Looking forward to seeing your evidence!
 
BTW, I'm surprised to hear that sex is not spoken of in
Western/Puritan/Judeo-Christian Civilization, as I seem to pick up
several references to sex every day.

Answer soon!

The best-argued contention goes: Because they were selected for that
preference.

Although indeed a marvel of engineering, humans are amazingly poorly
adapted for competitive existence when compared with other extant
organisms like tigers and other primates.  An adult male green monkey
(30 inches tall, weighing about as many pounds) can generally wrestle
into flight or submission an adult human male six times his weight.  A
60-pound baboon can do it every time, against every human.  On natural
terrestrial surfaces without trees or caves, a tiger can
catch/kill/eat essentially any single unarmed human.  All three of
those non-human species gestate faster than, and reach adulthood at
least twice as fast as, the human.  So why have humans lasted at all?

From a selective viewpoint, the chief distinguishing advantages of the
human are its two opposable digits (which vastly simplify toolmaking)
and its two large brain hemispheres.  Although much of to-day's human
society has developed from the oversized frontal lobes' abnormal
capacity for sequential and spatial abstraction, the strongest
particular evolutionary advantage conveyed by the human brain is the
ability to act in larger tribes.  (Ironically, this appears to derive
chiefly from the prefrontal lobes -- making humans'
frontal-lobe-derived generative and abstractive abilities a
breathtaking coincidence.)  The ability of the individual human
organism to act as a member of a larger clan than competitor species
can tolerate allows far greater specialisation -- which enables the
development, over dozens of generations, of significant variety in
cognitive approach and environmental response.  (Interesting side
note: that is also, probably, why substantive subpopulations of humans
seem to have been selected for what we term to-day as 'depressive'
affects and 'hyperactive' attention patterns.)

But in a large group of cognitively-enabled, social, sexual organisms,
copulation can be distracting to more than just those involved in the
act.  The collective reason why humans prefer to copulate in relative
seclusion is the same reason why human females (unlike those of many
other primate species) give no overtly recognisable signal that they
are ovulating.  Selectively speaking, that's an astonishingly stupid
omission if sheer numeric reproduction is the only goal.  The groups
of protohumans who probably didn't mind who watched (e.g.
Neanderthals?) were at a greater disadvantage than their mere cranial
capacity would indicate.  They likely possessed more 'savage', more
natural, more easygoing attitudes toward sex.  But sex's overriding
importance to the sexual human animal meant that a greater portion of
each organism's attention was called to that activity by copulation
all around.  (They also, probably, could not as readily abstract sex
from other aspects of life -- possibly a justification for the larger
Cro-Magnon [et al] frontal lobes and their greater
executive/selective/inhibitory capabilities?)

So the tribes whose members could more readily focus on making good
tools prospered, at the expense of those tribes whose members'
attentions were more often called to reproductive activity because it
took place within their sensory range.

An alternate (possibly complementary; keep reading) explanation lies
in the relative 'fidelity' of human mating couplings.  A good case can
be made that humans are just about in the centre of the primate
'fidelity' continuum.  In primates, you can draw a very strong
correlation between (a) the average male's
penis-size-relative-to-body-size and (b) the likelihood that a given
female will copulate with a male other than her established mate.  By
comparison with other primates, the human is about in the middle of
both the (a) and (b) spectra.  The argument goes that primates with
larger penises are those more likely to seek sexual coupling outside
established relationships (which are a feature of essentially all
primate societies).  It has been mechanically demonstrated that larger
primate penises are more effective at pumping competitor semen away
from the cervical terminus of the vaginal cavity.  Of course, the
unusually large brain of the human means that (1) bearing young
becomes an incredibly strenuous exercise for the mother and (2) a new
organism must lead its first years of life in a hopelessly dependent
state by comparison even with the other (laughingly-helpless) newborn
primates.  One might essay that nature selected only those human males
with a predisposition to support their mates and offspring for
numerous years following the copulative act.  One might also argue
that nature selected human females with an astute ability to choose
their supportive, and copulative, mates independently of one another. 
Particularly in light of the second argument, discreet copulation
would seem to be a successful, individually evolved propagation
strategy.

If you're Judeo-Christian, you might just chalk it up to God's
preferences (cf. Genesis).  If you're Muslim, you might take it as a
sign that lewdness is a sin in God's eyes (cf. An-Nisah).

An interesting follow-on question would be 'Why are all adult sexual
primates so unable to ignore the act when they can tell it's happening
nearby?'  I don't have a satisfying answer to that one, and probably
wouldn't try.

The explanation above is very good. The fidelity issue is the the most
often taught in anthropology and primatology classes. In many primate
societies, infanticied is a very common trait, expecially if another
male has "won" another female who currently has children. By killing
her children, the male is given a chance to inpreganate the female
with his own children, thus passing on his own genes. This is selfish
gene theory. Therefore, it is always in the mother's interest to mate
in private, so that her future (or current) mate will not know whether
or not he is the father of the females children, and will thus a) no
kill them and b) bring them up as if they were his own.

Dear thetapir,

I am not an expert on sexuality, although I've heard many of my
evolutionary arguments listed above.  What follows is my personal
opinion on the matter.

Conclusion: Preference for privacy is a social construct inasmuch as
marriage and monogamy are social constructs. [In fact, I could also
argue that marriage itself drives privacy….but that is another
argument.]

Assumptions
1. Today's humans tend to have sex for recreational or entertainment
reasons.
2. Humans are social/cooperative beings.
3. Sexual drive is very instinctually driven, especially for males.

Argument: 
If two humans were out having sex in front of say other humans,
specifically males, then since sex is enjoyable in front of others,
the would be considered an inconsiderate/antisocial behavior since not
all observing could enjoy in this sexual enjoyment at the same moment
in time.  (It's almost the same reason for why are compelled to offer
food to those observing us eat, especially if they are hungry.)   If a
couple does not offer to enjoy in the entertainment (or need) of the
moment to those observing, then we run into the risk of being
ostracized when the tables are reversed.  This follows because we are
cooperative beings.

----------
I would welcome for you to tell me what you (or anyone) think about
this.  I thought about this very question a while back, when I
attended a very sexually charged event in a highly "proper" gathering
in a foreign country.

Claudietta

...and I would like to add that I've been to Latin American countries,
where people are practically doing "it" in the parks.  I think that
the reason they don't do it in private (as is socially dictated) is
that they live with their parents and have no private place to do it.
And the reason they don't fully do it merely that they would be put in
jail or socially ridiculed.

I think that if it weren't for social reasons, we'd probably all be
out there doing it!

Claudietta