I honestly don't know what you mean by 'beliefs on sex'. I believe sex exists. It's not, like, a fantasy.
Anyway, as for where I learned about it, I have multiple sources. My parents have always been pretty upfront with me about sex, maybe because they're both doctors and they don't get embarrassed about stuff like that. I could always ask them things if I was curious, and they would answer me in an age-appropriate way. In my country, we also have sex education in schools, so I learned about it there, too, although that was mostly things like anatomy, what STDs are, "how to say no", and forms of contraception, but, being homosexual, an awful lot of what they taught me felt more than a little irrelevant.
I actually got a lot of answers about the mechanics of sex from the internet more than any other source, just by reading about it, but then I guess that's not unusual anymore. It helped me demystify sex. Having those dry answers helped make sex boring in my teenage mind, so I wasn't all that interested in doing stupid things or making mistakes.
I'm kind of old-fashioned in my relationships. I've only been with one person sexually, although I've dated other people since then. I've learned that I don't like pushing things into a sexual place too fast. I mean, if all I wanted was sex, then, theoretically, I could sleep around with anyone, and have it be completely unemotional, but that's not what I'm interested in. If I genuinely want to be with someone, then I'm going to wait until that trust is there, and until I know it's not a purely physical thing.
That's just me, though. I get that everyone has different physical and emotional needs. Right now, I'm not even interested in relationships at all. Too busy with other stuff to have that on my mind.