What if they are missing out on the best relationship ever by not talking to each other truthfully. The gf will either be in it or not, why wait until she is really into the guy to find out that he is poly and wants to add this woman. It's tricky for sure, I'm not sure either is the right answer. I think you will just have to go with your gut as you may have already decided to do. It might not be worth pursuing at all if the love isn't really there, just the lust for experience and to get this guy in bed. It's up to you really, I see no right answer.
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