When I fell pregnant with ds1, my (now) ex and I decided that our parents should all meet properly for the first time as they all lived away from us. So my mum and her dh were there, then ex's mum and her dh arrived. The atmosphere was a bit strained from the get go.
My mum cornered me in the kitchen about 15 minutes later and asked when my dad was due to arrive. I explained that he was ill so had to cancel. My mum looked very relieved.
My ex's mum was married to her driving instructor, who she had had an affair with behind ex's dad's back and was the reason they got divorced.
Turns out that my mum also had an affair with him about 6 years earlier and although it didn't break up my parents, it did cause a lot of trouble in their relationship.
Ex and I at least had the opportunity to explain to our respective dads in private.