The same reason some people are born white and stay white. Or some people have black hair and stay with black hair. They are born that way and they have no choice.
Honestly, why would anyone CHOOSE to be gay? They are outcasted, thrown from their homes, shunned, and SO many people are always telling them they'll go to hell and they are wrong and all these things. They are beaten, cursed at, and many killed. Who the heck would WANT all that?
this kind of thing tends to be a touchy subject, and is best answered indirectly with more questions. this is because any part of the bible can be used to "prove" any part of the bible wrong, and because so many translations exist. the big question here being why should you or your father wish to follow the bible word for word on this one particular thing?
would you be willing to have sex with your father? odds are no, at least I'd hope lol. but what happened with lot and his kids? would your father order the death of your brother if he saw him naked? again no but its in there. do you wear more than one type of fabric at a time? odds are yes and according to the good book thats bad, says so in the same chapter as the big go-to anti gay reference. should women be forced to stay outside the community while on their period? should blind people be forbidden to go to church? should farmers be forbidden from raising more than one type of crop in a field? should people who have negative dreams concerning god be put to death? should we kill anyone who even suggests worshipping another god? I can give sources for where each of these is in the bible if you like.
so, why out of all of these (and a great many more I assure you) is being gay the only one that is questioned with any seriousness?
The Bible also says "love your neighbour", meaning be kind to and love everyone. You'll find the Bible contradicts itself a lot, and it's because it isn't a dictator that states exactly how to live your life, and what to think. It's merely guidance, people can choose what to believe and what to live their life by. Obviously, many gays, (as well as many others) do not read/go by the Bible, because they have a different or no religion. They don't want to be "ruled" by something they don't even believe to be true. Also, it's not as simple as "choosing" to be gay or not. Just as one can't control/alter one's personality, one can't control/alter their sexual orientation. Some people, who are ashamed of it or don't think it's right, try to change it or hide it, but it doesn't change the fact that it's a part of them. Many people don't believe "Hell" or any of that sort exists, and even among Christians, its existence is so controversial (is it an actual place? A state of being? A frame of mind?) that it's impossible to answer your second question. I say, don't concentrate on details like this: celebrate individuality, let everybody shine! It doesn't matter if they are a man or a woman; if a person is capable of loving, that's a wonderful thing, and nothing else matters.