Grandpa Does Grandpa Things
If you thought seeing your SO’s parents in Spock attire or completely nude was embarrassing, you ain’t seen nothing yet. The first time the OP’s dad met his future wife’s parents, the dad (the OP’s grandad) panted the dad. Like, he pulled the dad’s pants down in front of everyone. Silly gramps!
We assume they all had a jolly good laugh at that. Meanwhile, the dad probably came up with different schemes on how to get back at granddad for years and years. But when his status changed from GF’s dad to DIL, that’s when his plans were put on permanent hold.