No, it is not Gay. Nor is it Bisexual.
It is just another way that a man and woman can get closer together in their sexual relationship.
BioDefenseGrad - there is plenty of this on FreeOnes if you want to look for it - It is a porn board, after all - e.g. Shanda Fay - http://www.filthycuteonline.com/mgp/forbigdaddy3/
The anus is one of the erogenous zones and both sexes can get pleasure from stimulation. There is also the prostate, as has been mentioned by a few people, that can only be stimulated be penetration, and manipulation of this gland can be extremely pleasurable.
To label someone as gay or even bi because they enjoy pegging or other anal stimulation is crass.
I think men (and probably many women) would characterize it as 'gay' because it is most certainly an act of male emasculation irrespective of what rationale or pleasure is reasoned out of it. Most men are not into being emasculated nor do I think most women are into emasculated men.
I believe our natural perceptions are that men who succumb to emasculation or things that emasculate them have some subconscious that portends of homosexuality. True or not that is the perception.
Who's ever been in the US military pre 'don't ask, don't tell' probably remembers the questionnaire which asked, "Do you have homosexual tendencies?" Whatever that means and rightly or wrongly I'm sure a majority would consider a man who likes anal penetration falling into the 'yes' category for that question.