Sexual filter effect.

Basically unless you are really hot there's a good chance that you are, to most sexual partners, a kind of unknown quantity- if you're average in looks then controlling for that unknown factor often skews the risk-reward ratio against the person who often decides it's not worth the investment. What if they are crazy or violent or w/e beneath the surface and all that for someone I find only moderately attractive and was tepidly interested in at best?

Well if another female goes for that woman instead of her, and presumably sticks around instead of running screaming, well then that ambiguity is crushed and the risk-reward ratio swings in the other direction; you become desirable as someone else took the risk for them and it paid off so now they only see the positives in you.

The effect is most pronounced from single women to married men.

Most women who experience this will not be able to articulate it as it often happens subconciously, they aren't trying to play any games or be uncouth. Of the few who can articulate this shift a portion of them won't admit it- if they ever say that then help them realize it's either this explanation or admit they are just trying to deliberately fuck over their sexual competitors/other women they don't like as a dominance/hierarchy thing. I'm sure that has to be the case with at least a few women.