Why are there Catholics that are against the idea of ity being natural (E.g. against gay marriage)?

I'm an ex-Protestant and ex-creationist but I still believe in Jesus and God. I'm slowly learning about evolution, and Catholicism has been on my mind lately, since Catholics accept evolution. But they also believe uality is wrong? How? There is implicit evidence for ity being brought about by genetics (Evolution), which is God-given, so why would uality be wrong, and why would God not want us to be happy in marriage? I still believe outside of gay marriage is wrong, as well as gay adultery though. I can see how that can lead to a sinful life, but gay marriage restriction seems irrational in Catholicism to me.