Because it's the most important thing in the world.
You are(and I am) primarily a biological creature. A biological creature that has an evolutionary history that consists mainly of the most sexually prolific of those who came before us. Those that didn't care about sex didn't really have kids. Thus, they were weeded out of the gene pool and only those with the genes that desired sex survived.
Evolution is concerned with only those who adapt enough to have babies. Everything after that is unimportant. Requiring food and shelter are means to an end. That end? Little squalling poop machines that have your eyes and your significant others chin. That's it. That's the goal.
Technological luxuries have led to us casting off our more base natures. We're more free to put procreation on the back burner, while still engaging in that biological drive to have sex. Sex is still ridiculously important to you, but having children comes with responsibilities; responsibilities that a lot of people find distasteful.
But it's more than that. Society has required of people that they preform their baby making duties within certain confines. Those confines are called "marriage." Why? Because the most health, the strongest, the most well-adjusted, and overall best off people are people that come from a stable, two parent family(specifically, a mother and a father). A child with a nurturing mother that spends her time taking care of it while its father is off providing the means by which the nurturing mother can spend that time taking care of it, is immensely better off than a child that doesn't have one of those or both. Evolution has selected for such a partnership, because of how insanely strong such a foundation is for the young.
Homosexuality, on the other hand, is deviant behavior. Regardless of if you believe that you were "born that way" or not, lusting after something that will not produce offspring is contrary to, frankly, evolution. If you're not making babies, you're doing something wrong. Creating children is the biological reason for the act of sex. This, along with humanities biological preference for people to "conform" helps people to stigmatize and reject people that aren't like them. Thus leading people to look down upon homosexuals, as they aren't "like them."
It's pretty simple, really.