What do you think about this case of a "public" display of affection?

When my immediate family gets together, my brother and his boyfriend are so very often having affectionate conversations with themselves (“my little baby!” or whatever). This is accompanied by kissing, hugging, caressing, etc.

Now, if I’m on the train, I don’t have much of an opinion if two people are kissing or cuddling or whatever—I’ll just read my book and not think much of it. But my immediate family is a group of five people, and the affectionate communication that goes on between my brother and his boyfriend is exclusive to them. This is why I find it rude. The remaining three of us are left to awkwardly start talking about something else.

I want to confront them about this, but I want to make sure that my opinion is justified. What do you think about this? What should I say to them?