Weird At Holidays

My son decided to bring his girlfriend to the cabin for a few days, and I was shocked when it wasn’t his fiancé. Heidi is a very attractive 26 year old who worked for my son’s department. The first 2 nights, I could hear them in the guest room getting it on. Then, the next day they had a disagreement. I think all the calls and texts from his fiancé triggered it.
Heidi didn’t want to spend time with him anymore, and it’s a 4 hour ride back to her place, so we ended up hanging out. Yeah, you know where this is going, we ended up swimming at the old rock quarry under the waterfall. We climbed up onto one of the rocks, and laid out our clothes to dry and proceeded to make love. No, it wasn’t that passionate wild fucking. I pulled her towards me, and she sat in my lap facing me and guided me in. It was slow. Almost motionless. As she ground in a circle, I just gently sucked her nipples and we did that for what seemed like forever. She started rising and falling on me and whispered she wanted us to do it together. When we finally finished, she just stood up and dove back into the cold water. I followed her in and we kissed under the waterfall.
It’s been almost a year now, Heidi moved in and we have a nice little farm growing Kush. It doesn’t matter to me, but my son is freaked out that he did her before me.