My mom always told me that when I decide to have sex, it is definitely MY decision. She refused to buy me a purity ring when I went through a phase of never wanting to have sex before marriage because she didn't want to be part of such a strict rule about sex for me. Sure, she'd have liked if I waited but she was more concerned with me being true to myself, and SAFE once I decided to have sex.
As a teen, I had to work on weekends but my family always went up to the land they own for camping and hunting (my family processes all their own meat.) She knew I'd have my boyfriend over. She never SAID so in so many words nor did she ever approve of it, but it was always a 'I know its happening and there isn't anything I can do about it so lets not talk about it since I know you are safe' kind of thing. When I was 17, she had left banana bread for me and my then boyfriend. Banana bread is a breakfast food in my family so it was pretty obvious the intentions hehe.
With that said though, I wasn't allowed co ed sleep overs unless the boys were in a separate room. In fact, I only had one on prom night. Boys slept on couches upstairs and the girls slept on a futon downstairs. I also had a boyfriend come along on a camping trip... he had to sleep in a tent with my brother rather than me. Yes, my mom knew I was having sex and accepted it as fact. She was happy to know I was safe and liked the guys I dated (including the ones I didn't have sex with.) However she wasn't interested in hosting sleep overs.
honestly, had she allowed them to sleep over, there is no way I could have had sex. It would have freaked me out for her to possibly hear us. Hell, I'm married with a kid and it STILL freaks me out when we stay at her place... and I know she's cool about sex! But, I'm pretty private about it. I can talk about it with my friends in great detail but I prefer no one but my husband to know when we actually have sex or to hear us during.
I don't think it is wrong to allow the sleepover. They might not want to have sex with you in the house and prefer the house to themselves for full privacy like I did. Or maybe they feel really respected that you understand their relationship is a serious one and not just a silly fling. Maybe they tell their friends 'ha, my mom actually LETS me have him sleep over so we can have sex! Does she not KNOW?!' but i'm guessing its more of a normal 'well, we've been together, we're serious, we've been having sex for a while so why wouldn't we be able to share a room?' kind of a deal.
I don't know that i'd do it... I can see myself more like my mom... not disproving of sex, but not interested in boys sleeping over either but I can definitely understand why parents might be okay with it. I think the initial reaction is worse than the situation actually is.