The idea that in any poly relationship, a man could "replace" a man or a woman could "replace" a woman is ludicrous. What are we, clones?
Just because 2 partners have a penis or a vagina doesn't mean one can replace another. The 2 same gendered people look different, sound different, their faces are different, their penises or vaginas are different, their heights are probably different, their hair is different, their family backgrounds are different, their hometowns are different, their hobbies are different. Even if their penises or vaginas are the same shape and size, or their breasts, ass, etc. (which is highly unlikely), they have different sexual skills. Sure, one man might have a better sexual skill set. But a woman might have a BETTER sexual skill set than a man, since she doesn't rely on PIV to get a person off.
I didn’t mean that two people with same type of biology were actually directly interchangeable. Rather, it is more likely that someone might be concerned that their partner is trying to replace them with a “better” mate with an equivalent package. People are used to comparing their attributes to others of the same gender identity -- "s/he's stronger/curvier/prettier/funnier/more charming than I am." Of course someone of the opposite sex might have more desirable qualities, but from a very young age we are just more used to comparing/ranking ourselves to our peers of the same gender.
Also, an aside..... I am surprised at all the generalizations that men rely on their dicks to get women off, or are unconcerned with women’s pleasure as a whole. Is that really still common? I had read the research that Ravenscroft had mentioned (about intercourse lasting 2.5 minutes), but figured (1) That has never happened to me, but if it did it wouldn't be a cause for complaint. I would actually love to have a partner who orgasmed on the early side during PIV instead of fucking me forEVER and making me sore and exhausted in the process; (2) Why does it matter to women if PIV is short? It's the other stuff that usually is more fun for women anyway.... (3) It's gotta be old research, right? I mean, don't people (both men and women) understand so much more about the science of the female orgasm than they used to? Even an inexperienced guy knows it's important.....Again, it totally surprises me that there are men out there who would cum first and then just roll over and sleep. Literally, even the guys who were kind of assholes in other departments have been super focused (hyper-focused, in fact) on making sure I get off.