Gay men are gay for the same reason you are straight. It's not a choice, it's just that they are only attracted to other men whereas a straight person is only attracted to the opposite sex. It's not something you have control over. It's not that they have anything against women, they are just simply not attracted to them in a romantic or sexual way.
That's like asking why are lesbians lesbian that's just how they are I mean we all are born a certain way.. I'm BIsexaul and I didn't chose it I didn't wake up and think I like girls and guys...I just was attracted to girls and guy..I actually denied it for 2 years before I actually accepted it
after reading your responses to answers I feel this needs to be said. (I'm not gay, so I take my opinions with a grain of salt): Gay men aren't denying that heterosexuality is incredibly practical (its practical for a man to have sex with a woman because then the woman might get pregnant, give birth, and keep the human race alive, etc.) And I imagine that the only thing "wrong" with a woman that would make a gay man not be attracted to her is that she isn't a man. Gay men are just not turned on/attracted to women in the way that they are to men. So if they want to be in a relationship where they are attracted to their significant other, they have to be in a homosexual relationship. Why are they attracted to men like this but not women? probably for the same reason that you have certain color eyes, certain color hair, and are attracted to women but not men. Its the result of your DNA.
I'm sure some people can choose but most of them have always been that way. I have a friend who never dated girls and I knew was always different. He came out about being gay and his family doesn't talk to him anymore. Why would he "choose" to be hated by his family? He was very depressed for a while until he got help but he is like a brother and I love him to death. I support him for something he can't control. I mean I can't just suddenly like guys that doesn't even compute for me so I know I'm straight. Could you suddenly get turned on by men? Probably not which makes you straight.
I blame Glee. No one can ever be born gay. You can be born a transgender but that doesn't automatically make you gay. That's just like murderers being born murderers or racists being born racists. Being gay is a choice whether you like it or not.