Maybe laws against homosexuality made sense once upon a time, or maybe they were always a bad idea. But I don't need to know that in order to establish that homosexuality should be allowed and accepted today

"Need to know" is a strong condition, and I probably agree with you that you don't need to know the relevant history in order to establish that a particular law is a bad idea. But I would also agree that if I'm actually trying to determine whether a law ought to change (rather than trying to justify my pre-existing belief that the law ought to change), trying to understand how the law came to be in the first place seems like a really good place to start.