it seems like there are two main issues here... the first is if you would ever let a child or teen of the opposite sex of your child spend the night at your house. for me, the answer is yes... it might be warranted because of the situation, like in the case of the OP... bad weather, late night, etc. or if there are children of more than one gender in the house and they are all friends, the situation is likely to occur frequently. that doesn't automatically assume that the child of the opposite gender is sleeping in the same room or bed, just that they are spending the night under the same roof. i think that this was more of the situation that the OP was dealing with.
it gets more complicated when you start talking about whether to allow planned sleep-overs, and especially when you are dealing with the boyfriends or girlfriends of your children. i don't think i'd have a huge problem with children of a younger age having planned coed sleepovers if that's something they want to do. i personally didn't have close male friends as a child, so it never would have occured to me to have a boy sleep at my house. with good supervision, a group coed sleepover has about as much chance as a single sex sleepover of turning into something inappropriate. by the way, i'm not gay, but my first "sexual" (in that i didn't think of it as sexual at the time, but do now) encounter happened at a group sleepover of just girls, about 9-10 yrs old, none of whom indentified as gay (or even sexual in anyway)... it was just something that happened.
anyway, would I allow the known sexual/romantic partners of my children to spend the night in the same room (and here, i'm hoping that i'll know enough about my kids to know their sexual orientation and if they're sexually active)? no, i wouldn't. not because i don't think they're finding some way to have sex, or because i don't want to "allow" it, but because i think spending the night in the same bed frequently adds a layer of intensity to a relationship that my kids don't really need to be experiencing when they're young enough to be living at home. i had several friends in highschool go through what were essentially divorces (in intensity, pain and drama) who had been practically living together at one parent's house or the other. and if you're going to allow your teen to sleep with her boyfriend or girlfriend one night, where do you draw the line? for me, it's just not a step my kids need to take while i'm still buying their groceries and doing their laundry.
basically, i'm ok with the potential for a bit of sneaking around or sexual activity if my child has people staying over (or is staying over elsewhere)... it happens, and i wouldn't want to ban the fun and bonding of innocent sleepovers for that. but i would have a problem with coed sleepovers escalating to specific sexual/romantic partners spending the night in the same room, that is, until my kids go off to college and bring their s/o's over for visits.