Why are people so against gays, and gay marriages?

Okay I know I might be too young to understand this little fit everyone is throwing about this gay marriage and gays in general. I mean…I know the bible (or what ever) says that it has to be guys and girls. But C’mon it’s only religion. It doesn’t prove anything about it. I mean if you think about it. We’re all sinners anyways. So why make something like that so big. And that brings me to say that: “I think gay marriages should be approved” But it’s all in my opinion. I mean…I’m honestly not really sure about my sexuality. I can’t tell if I’m either Gay or Bi. But I do consider myself straight. Just why is it a big deal to everyone? Do you support them or not?