A romantic relationship is limited (ideally...) to the two individuals involved in this relationship.
This means that the relationship is exclusive. (Again, assuming that others are not free to get in on the action).
Therefore when in a group setting, a good thing to do is to try to be as inclusive as is reasonable.
All things considered, we want to have inclusive relationships (says Pioneer Camp) as opposed to exclusive ones.
I think this is why I don't like enthusiastic displays of public affection.
Unless they are artful.