Well, I’ll guess I’ll be the voice of lone dissent then. No. Co-ed sleepovers are not appropriate.
YMMV, of course, but in my house, no.
OK, since you’ve dissented, you must be pressed (it’s the Doper way!).
At what age does this strict rule start to apply?
Once they start kindergarten. Opposite sex relatives (cousins, etc) do not apply to this rule.
Look, you guys can make fun of me and deride me all you like. This is my moral value and I’m sticking to it. You will note, however, that I am not trying to convince you to adhere to my moral code. I would hope you will offer me the same regard.
I gotcha. No pressure, I was just curious about the specifics.
So around here school starts on August 25th. Sleepovers on August 24th are OK? 
Good for you. It would never have occurred to me to make fun of you.
As I mentioned, in my parents’ house coed sleepovers weren’t even considered. I don’t think this was a mistake on their part or that my sisters and I suffered for it.
Don’t you think anyone’s ever gotten it on with a cousin?
I tend to agree with you, though. I went to a few coed parties/sleepovers in high school, once in spite of the disapproval of my parents, and some afterwards without their knowledge (no need to have that argument again. I never had sex at any of them (not for lack of trying a few times), but that wasn’t the case for everyone there.
That said, I don’t know that I’d be unwilling to chaperone such a gathering myself. After all, I know that I will be there to supervise.
Heh, You’re not alone. Just last night I caught my 7yo son (sort of) making out with his 8yo cousin. So, at least to me, the idea that kids are naive to sexuality is laughable to me.
But yeah, I guess I’d let the boy stay the night. But he’ll be sleeping with me handcuffed to me as well. 
If they are 9 years old it would probably be no problem. As they get older though, you do want to be wary, even if your daughter is staying at a female friends house, if said female friend has a similar aged brother. Back when I was 14 I lost my virginity to my younger (13) sister’s friend who was staying the night. Thankfully my parents sleep like rocks and I was smart enough to have pilfered a condom earlier. That friend of my sister ended up getting pregnant at 16 and I thinks she lives in a trailer right now.
Every female friend at whose house I’ve slept had one or more brothers close to our age.
I once slept over at a female friend’s while in college; we shared a queen-size bed and always referred to it as “sleeping at her parent’s” because if we had said “sleeping together”, people would have interpreted it as “having sex”. I hate euphemisms because then when you mean what the euphemistic sentence says, people goes and understands the taboo meaning.
There shouldn’t be a problem with your kid’s friend sleeping over, I think. If you can provide a separate room (is there a guest room?) or do that living-room camp thing, better… but at that age, my own notions about the difference between boys and girls was limited to “our school uniform includes a tartan skirt and theirs includes navy blue trousers”.