Let me put it this way: heterosexual women who enjoy receiving anal sex are no closer to being gay men than their male partners.
Put even more simply: No. There are no physiological differences between gay and straight men that allow one to experience heightened pleasure during anal sex (topping OR bottoming).
Rather, it is the psychological and sexual identity differences (which may, indeed, have a genetic or other biological component) that allow some men to enjoy giving or receiving anal. Note that I'm not making a strict gay/straight delineation here – there are plenty of straight men who enjoy receiving anal sex from their wife/girlfriend using a strap-on. Heck, it's a whole educational video series:
- Bend Over Boyfriend http://en.wikipedia.org/wiki/Ben…
In theory, everyone in good colo-rectal health has the physical capacity to enjoy anal sex. In practice, this is a lot more a case of "nurture" rather than "nature" determining the outcome – both in terms of sexual identity and being comfortable with the idea, as well as being comfortable with the act itself (usually involving cautious insertion and lots of lubricant).
With enough patience, practice, and prostate stimulation, just about anyone can come to enjoy anal sex. Er, pun intended.