The difference in your situation and anyone else's is that your first child to have a date sleep over is gay. The point should be that she is OUT and this is a person who your daughter is attracted to. You should think of her as the same as a boyfriend in terms of sexual activity in your home.
Your husband doesn't want kids having sex under his roof. I understand and agree with that. They are still in high school. They are still minors. You have younger kids in the house who are watching this. They are not old enough to be serious or monogamous. Yes, they will find somewhere to have sex if they are determined, but that doesn't mean you give them carte blanche opportunity to crawl between the sheets, mama.
You will have to deal with the issue with your younger kids. And yes, when they push you with THEIR agenda later on, they will say "But you let sis have HER girlfriend over!" and you will not be able to say "But that's different - she couldn't get pregnant." They will just come right back at you that with good birth control, they can't get pregnant either, and they will look at you like you're a hypocrit.
Now, the idea that no date can be at the house after 8pm is far TOO strict. THAT'S what you should be dealing with. I had a son with a girlfriend, and though they were allowed upstairs, they were not allowed to have a closed door between them and the rest of the house. Now, HER mother allowed that at her house, but I told my son that he needed to respect her mother and open the door to her bedroom HIMSELF regardless of what his girlfriend wanted. She's not his girlfriend anymore, but her family likes and respects him anyway because he was always a gentleman.
As far as spending the night together? If my son's girlfriend was living far away and needed to spend the night, that would have been okay, but she would NOT have been sleeping any where near his room.
You need to explain to your daughter that she cannot have her cake and eat it too. She cannot say that she is gay and have a girlfriend and then act like it's a "sleepover". It's not. There is no difference than if your daughter were a BOY and having his girlfriend over for sex in you and your husband's house at 17. It's ridiculous, mama. You will cause REAL PROBLEMS in your marriage if you foist this on your husband, and your other kids will think that sex is a free ride for them to.
Instead, you and your husband need to tell your daughter that they cannot have the girlfriend spend the night, period. If she lives far away, then she can sleep on the couch. If she doesn't, and this is just for convenience sake for leaving early the next morning, she can just get up early for you all to pick her up. Put your foot down now and tell your daughter that you aren't having sex in the house with ANY of her girlfriends.
Then get your husband to compromise about the hours that girlfriends/boyfriends can visit. Tell him that it's better that they're watching a movie together in the house than making out in a car or someone else's basement til curfew.
You mention breeding sneaky children, but that doesn't mean that you let them just do ANYTHING because you don't want them to sneak. Pregnancy is not the ONLY thing you should be considering here. There is real responsibility that should be tied to having sex. Allowing it in your home in front of the younger kids is NOT being responsible. And parading a string of partners in, which is also a real possibility down the line, is unacceptable. THAT'S what you will have with all of your children, if you don't change your way of looking at things. At what age do you make the cut-off? Your younger girls just go out and get the pill and then can come home and have sex in the bedroom as if they are married folk like you? How is it that you are teaching responsibility?
Let them go out and get their own place if they want to have sex. You actually call them CHILDREN in your post. Why would you allow CHILDREN to have sex in your house? I wonder what the parents of the other child would think of this...