A couple of years back I dated a girl who told me she watched porn pretty much every day, especially given that we were long distance at the time. We would typically sext as one of us watched it and got off at random times throughout the day during the time we were separated.
Once we were back together in person again, obviously we knew that we both watch porn, and having had a lot of time to talk about ultimate fantasies and such during the numerous sexting occasions, we were already pretty familiar with one another's kinks. Most of those conversations were about making a MFM threesome come true for her and a FMF come true for me, to a point where we downloaded a couple apps and started seeing if a good match was out there.
Since we liked the thought of it so much, after a couple drinks one night we were just in bed watching Netflix. Started touching each other and I said fuck it, let's put on porn. You pick the first video, I'll pick the second and we'll alternate. She was obviously down. I was down. She basically edged me for an hour+ as we watched and she played with herself with her other hand.
But in short, definitely need to be comfortable with your partner watching it, but it also became an awesome way to see exactly what your significant other fantasizes about the most by watching the videos the other searches for and picks out.