Ok, I would say that I would want to be introduced before progresses too far. If they go too far, then it could bring in that nasty jealousy bone. I hate that bone, but it still lives in me from time to time. It typically only rears it's ugly head when it comes to this one woman I'm trying to develop a closer relationship with. (My wife and I have been dating her husband and her off and on for a while now) So I think I would want to know right up front. Otherwise, it feels (to me anyway) like it's the beginnings of cheating and deception. I tell my wife EVERYTHING that happens in my life throughout the day. If a woman (or in some cases a man) hits on me, I tell her about it up front. Sometimes, she tells me to follow up on it. Others, she shrugs it off.
Sooo...Yep. Definatly up front.