I obviously don't know you at all, so can only share my own experience of this. My wife and I are, by anybody's definitions, old hippies! Both baby boomers, who were both 19 in 1967. Despite the pill, she fell pregnant and we had a daughter in 1968. Due to immense social pressure even in those swinging 60s, we married and our daughter was born four months later.
You can work out how old she is now but, despite making me a grandad three times over, she'll always be my little girl.
Despite that, she was always allowed to have boyfriends to stay over in her room from the age of 16. We felt that it would totally hypocritical of us to disallow this! We felt we'd brought her up with her head screwed on and had answered her questions ranging from the biological to the metaphysical to the best of our ability. My best piece of advice, I think, was "It doesn't have to be someone you love, but it does have to be someone you trust".
As far as I can reasonably ascertain, the number of boys she brought home was three, and she married the final one.
As you clearly know, he's not a child any more, so don't treat him like one. What if any other of your unmarried couple friends came to stay? You wouldn't make them sleep in separate rooms... :-)