They do say that girls with a step mum, especially at teenage hood, is the most fraught combination. So I do think gender counts for a lot.
Also, my DP, like a lot of Dads, is desperate to be loved and needed and leaving the marriage made him fearful he'd lose that with his precious daughters, so became super indulgent with them. As well as that, he let himself become manipulated a lot, and lost respect.
And although I wasn't the other woman, his Ex Wife passed on a lot of her grievances to her daughters about me.
My Ex is a complete pain too, but he never stirs up trouble with my DP through my son, never puts him down. And I am fairly strict with my kids that they have to treat other people well. My son wanted me to have a partner too, he was completely open to adjusting. The girls did not want to adjust one inch. I don't know how much of that is a girl/boy thing, or what happened in their lives previously?
So... I suppose there are a lot of factors.