What is a list of the best adult movies that are graphic in content
but that feature ?normal? sexual activity between monogamous
heterosexual couples? The term ?normal? can be very broadly defined,
but I?m expressly not interested in unrealistic superhuman sexual
feats (e.g., quasi-perpetual and routinely multiple orgasms, etc.).
Ideally, the recommendations would also have a fairly absorbing and
enthralling storyline. Finally, the movies should be fairly recent
(say within 20 years) and available on DVD.

Movies with enthralling storylines that feature monogamous
heterosexual couples? I'm not very well-versed on the subject, but
that doesn't sound like porn to me.

There are movies meeting this description which have a few
sexually-explicit scenes, but the movies I'm thinking of are not porn.
Consider the marvelous sex scene between Julie Christie and Donald
Sutherland in "Don't Look Now." It's very erotic, fairly explicit sex
between two people who are married to each other. However, the movie
as a whole is not about sex, and it's anything but porn.

Here are some discussions of "porn for couples" that might interest you:

http://www.clitical.com/sensual-senses/porn-for-couples.php

http://www.tinynibbles.com/pornforcouples.html

It's not a movie site but it meets your other criteria:
http://www.janesguide.com/frames.html
Go to the  "Review Index", then "Personal Pages, Couples"

There is also a book you might be interested in that was highly
reviewed by Jane's Guide.
The Ultimate Guide to Adult Videos: How to Warch Adult Videos and Make
Your Sex Life Sizzle
http://search.barnesandnoble.com/booksearch/isbnInquiry.asp?userid=yw5RcRwAcj&isbn=1573441724&itm=1

There is also "The Best of Real Sex" form HBO at Amazon
http://www.amazon.com/exec/obidos/tg/detail/-/6304441207/102-2618615-8741768?v=glance

I'm going out on a limb here, given it's valentines day and all - and
if I'm wrong and am making a jackass of myself - I'm sorry.  Just move
on pass this comment and say to yourself - he's a jackass.

With the disclaimer out of the way.  This sounds like, and I MIGHT BE
WRONG, that your girlfriend or wife, came up with this to make you
happy.  If that is the case, let me give you a little UNWANTED and
UNASKED for advice from a guy who has been married for a long, long
time and understands men. That's right, men.  Don't go down this road.
 Take your wife to a "chick flick" and a really nice dinner, buy her
some roses -- you will be way ahead.  The porn thing - not good.  Look
at all the porn sites on the internet.  Porn sites are not popular
with women, as a matter fact, women would like to have them shut down.
 Who goes to porn sites.....?  Men. Mostly men.  So...

Now there are women who like this kind of stuff - no question about
it.  And if that is the case, more power to you. And if she did like
this kind of stuff, she'd be buying it for you, instead of the other
way around.

I'd be really careful - think mine field, think "Sweetheart, you look
great in that dress - new girdle?"