Why do many gay men fantasize about taking a heterosexual's virginity?
Sex is often not about sensual pleasure, but about power. The will to dominate, cause submission in others, and bend others to your will is a trait shared by all humans, but the testosterone driving men of all sexual affiliations often causes this urge to manifest itself in an aggressive fashion.
Habits formed over millions of years of evolution are hard to break, especially because science shows that the physical evolution of the human has not caught up to his technical prowess or ability to mentally adapt to situations. In short, we are still cavemen with computers. Secular urges, born of the depths of Freud's id, remain the most biologically prevalent of human urges, because they are by far the most important. The main focus of our body is still to procreate, plain and simple.
In order to procreate, one needs sexual prospects, and the attractive actions we take in everyday life, from showers to going for that promotion, are to attract more of these prospects. For testosterone laden men, the urge to hunt down these sexual prospects and attract them into our fold is the still the strongest urge we feel. Once we recieve the ability to choose our prospects, as technology and contraceptives have made it exceedingly easy to do, our eyes wander. The need has been fulfilled. We are now looking for what we want.
The power to have what we want is an incredibly seductive aphrodesiac, both for the self and to any person looking at you. By definition, the sexual urges of gay men drive them to want men, and to drive a straight man to bend to a gay will is the penultimate proof of social, attractive, and sexual power. The urge to have what is hardest to get also comes from the caveman days, where the most succulent fruit was at the top of the tree and the animal with the most meat on it was the hardest to kill. As well, the most attractive sexual partners are the ones with choices. If a gay man has the choice to be sexually active with a straight man, and moreso the ability to attract straight men into gay action, his sexual choices go up. He gets himself off because his own power is validated. He has what is least attainable and so will gain not only sexual choice, but alpha status within his peer group, which brings social rewards as well. In short, he will experience a better life overal.
The straight man's dilemma of wanting the gay man and resisting those urges is a different issue, but deals with the same themes of power and sexual choice. A straight man who is openly sexual with a gay man will lose status in his peer group, and women will be less likely to mate with him. He loses sexual choices, and power, and thus reduces himself to a worse life. This still does not explain, however, from where the urges come.
Biologically, girls and guys are not separated by much, and everyone begins in the womb as a girl. The infusion of certain hormones and the changing of a few chromosomes (a very few) are the entirety of the biological separation between the sexes. Many experiments have concluded that most of the differences we call male and female are socially based, not biologically based. It would not be such a stretch to follow that human attraction is less biologically predisposed, and more socially injected. Straight men are taught to like women and not men just like they are taught to play with toy cars and not Barbie dolls.
Many of these straight men, responding to biological predisposition to be attracted to humans in general, must still socially consider themselves straight, as admitting to being homosexual or even heteroflexible is social suicide. For a straight man, this also reduces his chances to procreate. So the issue is not just social, but also genetic. If a straight man is anything but, his genes immediately lose chances to pass themselves on.
In certain areas of the world and in certain more understanding communities, alternative sexual behavior brings little or no negative social aftereffects. Without this social pressure, people are more likely to define themselves as heteroflexible. Especially when not satisfied in a traditional relationship, men in these communities are much more likely to respond to biological urgings, and have sexual relations with any human to whom they are attracted.
Our fantasies are controlled by our bodies. Our actions are controlled by our friends. To act on a fantasy, social standing must be preserved. As more and more communities become more open to alternative relationships, thereby removing the very real adverse social effects, you can expect to see more fuzzy definitions of sexuality.