I don't care if a guy I'm with LOOKS, because of course we all look. But I feel like it's disrespectful of my feelings to be super obvious about it, do double and triple-takes [lol], or follow them with his eyes right in front of me. Because I DO notice these things. It's even more disrespectful of my feelings to comment on girls he's looking at, to me. I don't care if he looks, I just don't want to see it, and I especially don't want to hear about it. Basically, it's all about the manner in which he does these things.
Out of respect for my partner, I don't do those things either. If I look, I'm discreet about it and no one even knows I'm looking. And I definitely don't make comments about the other person being attractive, or what attributes I like on them, because I don't want to make my partner feel bad or LESS attractive.
To answer your question, it makes me feel bad when my partner checks out other girls in front of me [in an obvious way, enough to make me notice it], and makes me feel worse when he has the balls to say something about how hot she is [or what body parts are nice] directly TO me. Especially if that partner isn't complimenting ME enough in general or making me feel attractive in other ways. If my guy is making me feel attractive and I have no doubts in my mind as to how much he wants ME, then it wouldn't bother me quite as much. But I've dealt with exactly the situation I described, way too many times, and it bothers the f.uck out of me. It's a good way to piss me off and make me not want to be around my guy for a little bit, lol, not gonna lie. I've been disrespected a lot, and I'm not going to sit back and act like I'm okay with it when I'm not.