Yes I would allow my child of around that age mixed sleepovers if she wanted them.
She is informed (in an age appropriate way) about sex, physical boundaries etc and what is and isn't appropriate.
We speak very openly about such matters (again in an age appropriate way) so I believe she would tell me if anybody behaved inappropriately towards her.
Of course I cannot know this 100%. But unfortunately she is statistically much more likely to be at risk from an adult male who is known to her, than a friend her own age on a sleepover. I can't protect her 100% from risk even if I was with her all the time - women and girls can't rape proof their lives unfortunately (nor should the burden be on us to do so).
But I think the risks here are very small. If the worry is more about mutual bodily exploration, then if she wished to do this she would find an opportunity to do so regardless of whether I let a boy sleep in the house (sexual behaviours do not only happen at night) and it might happen with a female friend as easily - not all sexual curiousity is heterosexual!
So I'm focused more on educating her so that she doesn't make unsafe choices hopefully. It's not bullet proof - nothing is.
My mom allowed me mixed sex sleepovers and to be honest I am surprised most people are so anti it - I didn't realise it was generally frowned on so much!
As for the girl being a boy thing, that is a whole other show I don't want to get into - but it wouldn't impact on my sleepover decision either way.