I take it he is worried that if he enjoys it he will be gay. If he doesn't find men attractive and doesn't lust after them, then regardless of what his body enjoys I wouldn't label him as gay.

More to the point, a man who doesn't like anal (it hurts for him perhaps) but is attracted to men is still gay. A man taking his woman in the ass is not gay, and I fail to see why reversing the roles changes anything.

Personally, I have enjoyed have my prostate stimulated, and the above is how I would explain my own thoughts and feelings about it.