"it's not just about the sex, it's about the intensity of serious relationships at a such a young age"
Ragwort ... yes, I agree.
I also have an old friend whose parents allowed her boyfriend, when they were both 18, to stay over at their house in her room. It got to the point where this lad was practically living at their house 24/7. The problem was that this situation changed what was really a hormonally-driven "Sweet Valley High" summer teen romance into something far more intense and serious, and normalised the idea, for my friend, that adulthood and sexual relationships automatically meant cohabiting with a boyfriend.
This ended up having cascading implications because when they both went to Uni in the same city, they thought it natural to just get a flat together instead of individually going into halls of residence (this idea again was supported by her parents).
The result was that neither of them made many new friends at their respective universities, they were both socially alienated from their new environments and, when my mate realised the lad wasn't really a good match for her, it was incredibly difficult for her to break it off because, at 19 years old, she shared a flat and bills with him and hadn't built strong enough alternative social support networks. She ended up staying with him for another year and a half in a climate of abject misery.
But it was that early normalisation of what adulthood and a relationship should be like, supported by her parents' assent, that caused the most damage. The long term implications, twenty years on, have been that my mate cannot cope with a) living on her own or b) not being in an intense relationship -- which led her to make some very odd choices when her marriage broke down. She simply never did that young adult freedom thing; from age 18, she has always had a pseudo-husband in tow or waiting for her at home, and it has affected the development of her sense of self and independence.
Of course, I am not saying this happens to every young 18-year-old woman whose parents allow a boyfriend to regularly stay over, but I have seen enough awkward consequences over the years to suspect it is not such a good idea.