Hi, and good luck.
Yes, a One Penis Policy is sexist. You had a bad exp. with a former gf and metamour (the other guy). That doesn't mean if your wife ends up dating a man at some point, it would cause her to treat you badly.
Secondly, her "offering" you a threesome with a hypothetical gf is talking from a place of couple privilege. "Bringing the new woman into your marriage" is a misnomer. More likely the scenario would be, does your wife end up with a steady gf? Do you like her gf (your metamour) as a friend? Does she like you as a friend? Then, is there any sexual/emotional/romantic attraction between the two of you? Is it mutual? Then, and only then, would a relationship of more than metamours happen. It has nothing to do with your wife, it would be between you and the other woman entirely.
Then, do you want one on one sex with her, or do all 3 of you want threesomes? Sometimes? Every time? Once or twice a year? Once or twice a week or month? All the stars need to be aligned.
My next opinion is on your wife's first date. How very odd that her new potential's husband AND his gf are coming along on the date weekend! What the heck? Spies? Bodyguards? Sexual stimulation for them? WHY?
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 61) loving miss pixi (poly, F, 39) since January 2009, living together since 2013
"Master," (mono, 36), miss pixi's Dom for 3 years