I don't think they can smell it unless you have some dog sperm left on you. If you're having sex while there's another dog in the room, and he's also interested in you, he will usually wait because dogs have a clear picking order. The bitch decides whose turn it is, that'll be you
Knot is not very painful as long as it's not a huge dog. What can be painful is when he turns around and you're stuck, butt to butt, and he's trying to pull it out. In that case just try to grab both hind legs and pull him against you, until the knot shrinks and he slips out.