In a dinner party, does everyone at all times engage in conversation with the entirety of the rest of the group? At every dinner party I've attended, people will occasionally converse with one of their neighbors, and sometimes partake in the general group conversation.
If it's not rude to occasionally exchange a few words with the person beside you who is not your SO (provided, of course, that you are not ignoring the rest of the group for the majority of the evening), why would it be wrong to do so with a significant other (with the same caveat as before)?
Is it required that all people with significant others behave in public as if their SO is not their SO? Because that's the vibe I'm getting from many of these posts.
This is the behavior in question:
- When sitting around the table eating cake, Zoe dragged her chair right up close to Zac's, and nestled into his body (head resting on his shoulder) whilst she ate her cake;
- Later, when sitting on the sofa drinking coffee, Zoe sat so close to Zac that she was almost sitting on his lap. She also spent the entire time stroking his knee. When she wasn't stroking his knee, she was holding onto his arm.
- A couple of times, Zoe and Zac disengaged from the main conversion, and had a brief (lasting only seconds), whispered conversation of their own, which ended with Zoe giving Zac a quick, soft, kiss on the lips.
If the behavior as mentioned was constant through the entire evening, pulling them away from paying attention to everyone else, then yes, that would be rude. But it would be rude if it were anyone separating themselves from a conversation regularly. A couple few-second conversations concluded with a quick kiss, sitting close to a SO and touching them in a non-sexual way, and eating cake in a very inefficient way (C'mon, cake should be better appreciated!) don't seem that bad, unless the visit was a very short one.
And as a person who occasionally kisses her husband in public, who enjoys sitting beside him with her arm tucked into his, and who will rest her head on his shoulder if she's tired or headachey... being approached and told that my behavior was rude and making people uncomfortable would make me feel lectured and unwelcome.
I have often seen it said here that actions have consequences. That is true. Their behavior, though not bothersome to me, was unacceptable to the OP and her mother. As a consequence, they might no longer be invited over. That's fine. But keep in mind that informing the friend of this may have the natural consequence of the friend no longer wanting to be invited anywhere. These would all be people acting in polite ways, just with varying levels of how much PDA or judgment they are willing to bear to maintain a friendship.