Why anyone thinks their sex life is more important than their children’s peace of mind is a mystery to me
The two aren't mutually exclusive.
Mothers who are still with their DC's gather still have sex, you know.
Fathers can be bullies and abusers too, you know.
If have someone's child after, say, two years of being with them, how do you know they're any more trustworthy than someone you meet post split? Who you'd like to move in with after, say, five years together, but you won't countenance it because the DC are still young and at home?
Why is a man's prospective behaviour not a concern just because he happens to be the child's biological father?
Lots of bio fathers are absolute horrors to their children (and partners). Many stepfathers are loving, good parents (like my DH).