Because of personal orientation.
I fully believe there's more to being gay than sex acts. I don't understand why people don't acknowledge the obvious: gayness is more than fucking people of the same sex; it's a different way of being than heterosexual male or female ways of being.
I am obviously gay. Looking at home movies from when I was three years old, I was obviously gay. It was obvious by my movements, by my sensitivity, by my interests. I didn't have a sexual awakening until around age 12, yet it was clear to me that I was different than other boys, and it was clear to adults that I was most likely gay. (Although, as my mother always said, there was 'some possibility you were just an intellectual. A lot of them act gay even when they aren't.')
We all know what stereotypical gay mannerisms are. They're called effeminate in men and masculine in women because they're different than how straight people move, speak, etc., but they're also not exactly the same. Drag Race demonstrates this: a lot of gay men who are regarded as very 'effeminate,' once they put on women's clothing and makeup, should be indistinguishable from women in their behavior, but they are not; they're recognizably still gay. Gay men speak differently and move differently than both straight men and straight women, and the same is true of lesbians. A 'dykey' lesbian acts differently than a straight man—she just doesn't act the same as most women and so her behavior is called masculine, but it is not the same as straight male behavior, speech and thought.
So the first point is that gay men are not the same as women whether we are effeminate or more 'straight acting.' (Most gay men I have met who regard themselves as straight acting are very obviously gay both in voice and movements, by the way.)
Sexually speaking, to generalize, gay people are physically attracted to the same sex. Gay men, regardless of how we act, have the same anatomy as straight men and so there's no reason not to be attracted to a male body embodied by a gay-acting guy.
Beyond that, I think social psychology comes into play. Being a straight man still is perceived as more normal and people want to be seen as normal. All messaging we get all our lives about men is that masculine men are sexy and 'weaker' men are not. Most 'straight acting' gay men, who mostly are not straight acting, exercise fanatically to have a sexy 'masculine' body and they believe this makes them closer to a masculine straight ideal, and that is funny to me because as we get into adulthood most actually straight men stop working out entirely and become soft and squishy, while most adult men who have 'perfect' 'masculine' bodies are gay. A lot of psychological complexes and homophobia remain in our culture regardless of changing laws, and many gay men feel shame about being different than the norm for men and spend their lives trying to identify as and with straight men. Gay porn, ubiquitous now, reinforces these insecurities with 90-95 percent of storylines being about two straight men fucking, a gay guy coercing or blackmailing or otherwise tricking a straight man into sex, or seducing a straight man on the 'down low.' This has also created a ubiquitous thrill in hunting and trapping elusive prey—the satisfaction of conquering and converting the unwilling. I think a lot of religious fanatics obsessively seek out the same payoff in trying to convert people into their cults. It may be the same base psychological drive with different end goals.