A survey conducted by the University of Texas found 237 reasons. According to this article in the NYT, “They even found a few people who claimed to have been motivated by the desire to have a child.”

The number one reason was “I was attracted to the person.”  On the other hand, defensive sex is also popular:

[B]oth sexes seem to practice a strategy that he calls mate-guarding, as illustrated in one of the reasons given by survey respondents: “I was afraid my partner would have an affair if I didn’t.”  That fear seems especially reasonable after you finish reading Dr. Buss’s paper and realize just how many reasons there are for infidelity.