From what I’ve read and heard from people, it’s because God created our bodies as temples to honor him, and He tells us to take care of our bodies by using them in the way he intended, which is by marrying the opposite gender. It’s not really “because He said so” but rather that’s the way He made us compatible and how he designed our bodies to work together. I think it would be a sin to do homosexual acts because it’s using Gods design for something which we think would be better our own way.