To be honest, I can understand why you want to do this, it's potentially quite comforting and you're both exhausted after sex. I don't think I've ever fallen asleep still 'conjoined' with my boyfriend, but we've come pretty close, he wasn't on top though, being a petite little thing and him being a bulky climber I think that might have killed me. As far as I know, it's safe, though as some have pointed out, as you shrink it will probably come out.