I think it's strange to think that a 12yo having a "boyfriend" is going to be about "premature sexualisation", unless the boyfriend is significantly older. Unless children are getting a lot of odd peer pressure (and I think you'd know that from their other behaviour) the kinds of things they will do with their boyfriend/girlfriend at 12 are not that different from what they do with a friend - they hang out together, it's more about a label than any sexual activity.
As they go through puberty, yes there will be an increasing sexual element, with snogging and a bit of groping gradually creeping in. But they don't all start having full sex at 13 just because they have been "allowed" to have a relationship. A few will, but that's more about low self-esteem and worrying exposure to porn rather than it being considered "normal". And that hasn't changed since we were teenagers, there were always a few people in any school who had chaotic home lives and had sex for bad reasons.
I think teenagers have far more chance of gradually learning about themselves and about relationships by being "allowed" to try them out little by little, with reasonable boundaries about being home by a certain time, not having the door closed when they are together (certainly at 13 or 14), not having boyfriend/girlfriend stay over in the same room, etc, etc. That way they can explore their feelings, and the physical effects of them, in a safe environment, and not behind the bike sheds, or while drunk at a party you didn't even know about, or in any of the other places they will find, with people you have no chance to meet or hear about.
And when they do have issues, or want to tell you about "something that happened to a friend of a friend" you will be able to gently steer them away from dangerous behaviour and get them to think about why people let themselves be pushed into something they were uncomfortable about, or how they might protect themself from awkward situations.