Please make sure your kids don’t find out.
My dad had affairs for most of my childhood, my mum was like you, pragmatic about it. Content with everything else and didn’t want to upset the apple cart.
Still, people find out, rumours spread. My tenth birthday was ruined by a confrontation outside on our doorstep (the ex husband of my dad’s then affair partner). He lost a job over an affair with a colleague.
After 27 years of marriage he met a much younger woman and bought her a car. My mum asked him Wtf he was thinking and he left, no warning, just packed a bag and went.
A few years later he married a woman younger than me, his youngest child.
My mum died of cancer around the same time (in her early 50s).
I don’t talk to my father at all any more.
My childhood looked pretty good to outsiders, but me and my sister always knew that out father’s head and heart were not 100% with the family unit and we’ve had to work through the lasting repercussions of that through therapy. With risk of sounding like a cliche, this sort of set up can really leave a girl with daddy issues.
I can accept that your current situation suits you, but if you found out, it’s possible your kids will too, or her son will and one day he’ll decide it’s shitty for his mum and show up on your doorstep to tell your DH exactly that.
Plus your sexual health is at risk.