It's probably a comfort level thing.
I've slept with lurkette almost every night of our eight married years, and it's obviously no problem at this point. But occasionally we sleep in the same bed with another couple that is dear friends of ours, and I usually have a hard time falling asleep in that situation.
Of course, they both snore and I've totally got the hots for her, so both of those things sort of distract me from sleep...
