How is AIDS spread, other than getting it from someone else or through using used needles?

I'm so confused. I don't understand how AIDS was started in the first place. I read a study saying that AIDS started in gay men. I already know that HIV/AIDS is spread through sexual intercourse with an infected partner, and I already know that it can be spread through using used needles as well, so PLEASE don't regurgitate that information to me, I just want to know HOW AIDS was started and is it possible to get it from having unprotected anal sex with an UNINFECTED partner? I'm a girl and my boyfriend and I have anal sex sometimes, and neither of us are infected. It just seems that my gay guy friends are always getting speeches about protecting themselves from AIDS while straight people are not?