I think that after a while you kind of feel like a third wheel in a sense. Like, if you were to just hang out with other friends, it would feel comfortable. However, if you were to hang out with the lady friend and her love interest, there's kind of a feeling like you're less wanted.

Also, sometimes I think it has to do with the attitude of the potential new boyfriend/girlfriend toward their partner having opposite sex friends, as some get jealous very easily. I think it's a fear of the status of your relationship with these women changing, and I think it's pretty valid.