As you get older, you incidentally reveal yourself to women as either being a Loser- or someone who is going someplace.
You reveal yourself as either a fukboy- or a man who can make a commitment.
As you get into your 30's women can better assess your mating potential by whether you have a real career, whether you are willing to have kids, whether Other women have been willing to take the risk of relationship with you and a whole host of other clues that women are selecting for in mating choice.
Some women who have a safe relationship will nevertheless want to have a child with another man and the best candidate would be someone who might have the means to support the child should her primary relationship.
Some women are looking for a bigger better deal, and if you seem a better bet, they are not unwilling to break up a marriage.
And some women just feel safe flirting with a married guy- and yet- they are basically trying to identify potential future mates should Your marriage fall thru.
When you were younger- any woman showing interest in you was making a wild guess on whether you would end up a solid citizen, or a perpetual party boy or video game couch dweller.
Lots of young women are just looking for a fun guy or good time... which you might not have been.
But as they get older, women become more serious about looking for these indicators of stability and security.