People have aesthetic preferences. If I see two fat old guys going at it on the train then I'm going to find that pretty gross and not want to watch. If I see two hot young girls making doing the same then I'm going to be less revolted.
I'm sure you yourself find two attractive people going at it more pleasant to watch than two ugly people. That's just the way people are constructed. I don't think it's discriminatory unless you try and restrict the rights of one group in favour of the other.
Of course, we all have our preferences. I'm not going to pretend I'd find it pleasant to see two fat ugly dudes making out. I wouldn't. The thing is, though, I'm not going to even allow that situation concern me to the point where I'd feel revolted. I'd block it out if I wasn't liking what I was potentially in line to see. That's where the big issue is. It's like some people can't help themselves, and must let it be known what they "want" to see or don't want to see. And go out of their way to make an issue, make comments, call management/security in public building, etc. They don't let it go and ignore it, they turn it into an issue. In many cases, a persons' radar is more focused on the people they find unappealing, and that's a reality.
Yeah, obviously, people engaging in PDA which we individually find attractive, for whatever reason, will not create the same negative reaction within ourselves. But that doesn't mean that one should complain or embarrass those we aren't into on a visual level. If a person doesn't react when two women they find hot are engaging in PDA, then that person should have the respect to not react if 2 fat old dudes make out. Just keep it moving. Like you said, it's restricting the right for one group to do the same another group is allowed to that is discriminatory.