Everyone is different...
I feel that anything that is past a "family-friendly" level (light kissing, arms around the person, hand-holding - stuff you'd do in front of your mother) is something that I would consider intimate between me and my partner and therefore, I don't really want to be present when other people are doing these things. Doesn't matter if it's Chops and Xena, or other friends of mine. I get uncomfortable with it, like I'm viewing something I shouldn't be (and don't want to be), and just want to give them space. Hence, I'm not big on any PDAs of my own that aren't "family-friendly" either.
However, there's another facet of this which is feeling like a third wheel, and that's a bit of a different animal. Regardless of what level of affection everyone's comfortable with, if someone feels like they may as well be invisible around you, it's not a good feeling. The two can be related, but they don't have to be.
I try to make sure I'm not alienating anyone - when I'm with Chops and any of my metamours, I curb the affection a bit, for the reasons above. When I end up feeling like I want to leave the room because they've gone "all gooshy" for each other, or are playing a game of grab-ass, I tend to get a little out of joint.