When I was 13 years old I was sexually molested by a 25 year old man. 
I have never told my parents nor have I told anyone else.  The problem
I have now is that I have a wife, and occasionally I fantasize about
having a sexual relationship with a man.  I don't understand it though
and I have to be highly excited to make it turn me on.  Does this make
me gay?  I would never purposefully have a sexual relationship with a
man.

No this does not make you gay. Some times same sexes will fantasize
about the same sex.

I would have to agree with david1977.  To offer a counter view- as a
gay man I often fantasize sexually about women.  This doesn't make me
a heterosexual.  Most of my friends- gay, straight and bisexual have
said that their fantasies encompass the broad range of human sexuality
and expression- even though they only feel comfortable acting on a
much narrower part of the range with either opposite or same sex
partners.  As a man, i don't haveany stigma related to fantasizing
about women- even though I am gay-many people would even encourage
these hetero fantasies and hope that I act on it and "Convert".  As a
straight man- it's probably a little more difficult to find youself
having a homosexual fantasy and not have some shame or stigma
connected to the fantasy.  I don't fight my fantasies- I let them
happen when they do and I don't make anything more out them than they
are- fantasies.  hope this was useful.

You might find some useful information at this site:
http://www.pfox.org/asp/newsman/templates/newstemplate2.asp?articleid=123&zoneid=14

While a lot of people feel pulled or attracted to the site referred to
 by byrd-ga - keep im mind the fundamental premise of the "ex-gay"
movement is that homosexulity is a disease/sickness that can and needs
to be cured.  While this is completely conrtrary to our current
medical and psychological understandiing of homosexulity-
homosexuality is no longer classified or treated as an illness to be
cured but, merely a normal part of human sexuality.  This has not
stopped "experts" who mix science with their regious morals from
creating what most therpaists, psychiatrists and doctors consider to
be pseudo science with a program of therapy that is often deepens the
pain and denial the recipient beyond the initital pain they were
experiencing.  There is a painfully funny movie called "But I'm a
Cheerleader" that lampoons the "Ex-Gay" movement and it's process to
cure homosexuals.
Another avenue you may or may not want to pursue is to find a mens
group facilitated by a therapist where you could find a safe place to
discuss what you have been experiencing.  Sometimes just being able to
express yourself without fear of reprisal can put things like your
fantasies in appropriate perspective.

" Be who you are and say what you feel, because those 
 who mind don't matter and those who matter don't mind."
 --Dr. Seuss

No, it does NOT mean you are gay.  Your first sexual experience can
"wire you up" and that is one of the reason it's very important that
the first experience be, for a want of better words, loving and good.

An example to explain the above, a buddy of mine's first experiences
were with prostitutes.  Not loving and good as I view it. To this day
he battles with those feelings he had the first time.

As with you, dancingbear said it best and so does David1977.

If this issue becomes to heavy to carry, please get professional help.
 The fact that you were abused and have not told anyone, can be a
burden that many, many can not carry alone.  Any professional you seek
out, will keep your secret.