I see no problem with it at all. I'd be more than happy to have DS's female friends sleep over or have him sleep there.
I never asked for boys to sleep over throughout primary school as I wasn't great friends with any boys (beside my cousins) but I was allowed to have my male friends sleep over in High School. They did sleep in the loungeroom which I totally agree with at that age but that guy's lost his marbles if he's objecting to a 5yr olds sleep over!