As a former commercial airline pilot who retired for the industry in 2008. Yes, flight crew do hook up with flight attendants frequently. Not only do pilots hook up with the attendants but also passengers they meet on the flights, random women at bars hotels etc. Same goes for flight attendants, quite a lot of whom have multiple guys they see in different places.
We were warned in training how often this happens and that in smaller companies, assume everyone will know about it. It’s a stressful lifestyle and a lonely one, prone to a lot of drinking on layovers. Also, it seemed like most of the pilots who were married- were actively cheating or trying to cheat on their wives. I learned that one airline, the pilot’s wives came together to pressure the company into booking different hotels for Attendants and pilots to avoid this.
My Son is now an airline captain, I spent a lot of time hanging out with professional pilots, now i have retired and this has been being my impression. I shared an apartment with him and partied with his pilot buddies, and I can testify that these guys and flight attendants hook up all the time. As a general rule, they all liked to drink and have sex, a lot.
There's plenty of pilots and flight attendants hooking up for both long term relationships and one-night stands. There are also cases of pilots having double lives and two families (in Spain and in Miami or Cuba or some South American country) back in the days when the flight crews had to stay at their destinations for a few days. Neither family knew about each other. that was back min my day.