Two of my sims are suddenly gay and in a relationship with each other

Autonomous romances was more a thing in TS3 - with a disregard for gender. But if unplayed Sims do enchanting greets there is no telling who they'll end up with. Also, mods featuring story progression will create all sorts of couples. It's true that unplayed Sims will develop random friendships and enemyships that you'll come to learn of when you return to them. This is a thing.