Talking about the sexual or other strengths that one partner has to another partner
One of the rules I've created for myself that makes a lot of sense to me and, I think, has avoided me a world of trouble, is to never talk to one partner about the sexual (or other) strengths or weaknesses of the other partner. For example, it seems ill advised to me to tell my husband that he's a better lover than my boyfriend. Sometimes one or the other will fish for this kind of information and I gently but firmly stir away from it. That's not to say that both haven't acted as my friends and confidants at times of relationship stress (more my husband than my boyfriend); however, I try to be very careful about defining either one as in some concrete way better or worse than the other.
My reason for this is simply because, if the situation was such that I was one of multiple partners to my partner and he talked to me about her strengths or weaknesses, I would dwell on those conversations in a self-destructive way that wouldn't in any way help our relationship(s).