It sounds like Dan's not totally on board with poly but has acquiesced, even though it is painful for him. Wouldn't some compassion be in order, especially since this is a relatively recent change to your relationship? Two months is not very long. Is your fiance worth that considetation or not?
If I were you, I would ask myself which deal-breaker is more of a risk - the one that could prevent you from getting jiggy with whoever might come along, or the one that hurts your partner of six years.