People don't choose to be gay, they are born that way, think about it, all your life (if you are straight anyway) you were attracted to girls/women, why exactly? And why are girls attracted to guys? It doesn't really make sense, yet for some reason we still are, why? Because it's in our nature, now with my examples before nature isn't perfect, errors are made, things go wrong, you know no one is perfect, question is ofcourse what is perfect, it's something we don't really know, still what we know is that if everything goes right then you are straight.
Now it doesn't go always right, so someone is born gay or bisexual, they are attracted to the same sex or both sex's, now we can tell them go and act like a straight person, but would that make someone happy? You know my father has a friend and we are 100% sure he is gay, this guy is about 60 now, he has lived his whole life alone, why? Because when he was raised being gay was simply not accepted, and even though times change now, a part of him still lives in that time, and he will die without ever having a relationship, now that's sad.
When someone is born retarded, or can't walk or anything, then we try anything to help them, they get care, if they can do certain jobs then we adapt things to make it for them to be able to work etc, but when they are gay for some reason people still try to make it for them real hard, you know they can't help it that they are gay, and so what if they are, do they hurt you, do they hurt anyone close to you? Gay people simply get in a relationship with another gay person, or a bisexual person ofcourse, and they have a normal life, just like everyone else, sure they are of the same sex, and that's not how it's supposed to be, but many things aren't, but what is supposed to be is that we have a happy life, and if they are happy that way then who are we to tell it's wrong?
Now marriages were there to connect 2 people, it was never about connection 2 straight people, they always said to connect 1 person to another, and that's how it should be.
Just ask yourself this, is it fair that 1 person can't marry a other person just because they are off the same sex, is that really fair that you deny that to someone just because they are born that way even though they don't hurt anyone with it, and it makes them more happy?
Btw for the people who think you can become gay and that your not born with it, it's not a fact yet that you are born with it, but they done alot of checking with many thousands of people all over the years gay, bisexual and straight, and they can find no connection to why one is gay and the other is straight, and they checked for everything, parents, the role of their parents, their friends, brothers, sisters, other family, where they grew up, how they grew etc etc, they can't find anything.