Ime the married man usually presents himself to the single woman in a way that makes him technically married but actually single - the usual.ways are;
Together for kids, too.kids are older
Together for financial reasons
Together because she has MH problems/is vulnerable (often with implication not stable parent)
Together because parents, family etc would go mad, disinherit, whatever
Together until ABC timeline.
Like housemates, spark gone, biding time etc etc.
There is nearly always a narrative that makes him only technically married a d also makes him a good guy for not leaving her yet (would upset/destabilise kids, would destabilise her etc). Of course if he was a real good guy he wouldn't be sniffing around and getting involved with another woman while he's being such a "martyer" - he'd keep himself to himself til they split but ...
The narrative, alongside the fact that many women project their female motivations into cheating men (wouldn't be cheating if not unhappy and uncommitted) is what usually draws the single woman in.
One drawn in and engaged emotionally; the pick me dance/competition and her self esteem getting tangled up in why this man won't choose her completely and leave this woman for her, means they can get quite invested, addicted even .. to trying to "win".
If they're dumped, they are often very angry and very upset. Naturally - be wise they've most often been deceived, manipulated and future faked.