OP - please don't flounce - people are just trying to give you a bit of perspective here and reassure you, believe it or not! Unlike you, most of us were once teenage girls. 
Although you clearly didn't mean it in the way it came across, it did read as if you were making some unfair and out-of-the-blue value judgements about this girl of whom you know nothing.
She may be more experienced than him, or (especially given that she's younger) she may not. When I was that age I was totally inexperienced and assumed all blokes were worldly-wise James Bonds, wooing and bedding women with abandon. Clearly I was much mistaken. 
The point is - you know nothing about this girl, and the chances are far greater that she's as inexperienced as he is, that they're both caught up in the romance of it all (you know how teenagers think they invented everything and are the first to do/discover everything) so looking in from the outside it probably appears way more intense than it actually is.
As for her sleeping over, you're right not to condone it - she's only 15 and it would be wrong to do anything other than not allow it.
If you can bear it, have a chat with your son and find out where his head's at and make sure he's fully clued up about the responsibilities (not just the fun bits) that come with a sexual relationship - age of consent, actual consent from partner, contraception, feelings and emotions, trust, STIs, pregnancy, whether sex means that exclusivity is presumed, etc, etc, etc...