I may have a slightly different take on this.
My parents didn't let any of my boyfriends sleep in the same bed as me in their house until I got engaged in my mid 20s. Boyfriends had to sleep on the sofa downstairs if they were allowed to sleep over at all.
Their reason for this was that any sexual or intimate relationship I had was my business and until I made a formal declaration, they would not formally recognise it -- for the good of everyone concerned. They were always pleasant enough to anyone that came over, of course, but there was a line.
My mum told me that the reason they took this approach was that their early courtship had been made very awkward and difficult by my GP's relationship with my father's ex-girlfriend, who, while my dad was working away, had developed a very close relationship with them... sleeping over, having regular meals with them, going out for the day etc. When my dad didn't want to see her anymore, my GPs put an enormous amount of emotional pressure on my dad and refused to accept my mum, a situation that lasted for nigh on twenty years.
I do think that parents need to be careful not to get too involved in DC's lives and relationships. My aunt allowed my cousin to have her boyfriend stay over in her room when she was 18, and I can't help feeling that it made the relationship seem a lot more serious to my cousin than it really was. When he dumped her in a very nasty way, she was devastated, and my aunt felt horrendously gutted that she had been so accepting and kind towards the lad in her home, which she felt should be a sanctuary for my cousin from the nastiness of other people.