Different reason because you dont want to end up in a hospital ten times. Maybe two but seriously I wouldn't dare getting pregnant ten times, not even five. Maybe 3.
any sort of vibrating toy, or an orgasm and men who love dirty girls ;)
personaly i like one on one because there is more action envolved and less jealousy. more cock and pussy vs fighting over cock and pussy. Though it must be pleasure for the guys i must admit. I've been in a couple 3somes and have enjoyed them but over all i'd rather have one on one.