Greg responds...

Kissing is a human trait. Goliath and Demona didn't kiss in flashbacks because it was a custom they did not share. The gargoyle equivalent is to stroke the hair and/or horns/ridges of one's mate.

Angela & Broadway kissed because each had "endured" a lot of human influence. Broadway through pop culture (movies, tv, etc.) and Angela because she had been raised by the human couple Katharine and Tom (who did kiss).

Goliath stroked Elisa's hair more than once. But he never kissed her. She kissed him.

I don't recall Coldstone and Desdemona kissing. If that really happened then the truth is it's a mistake. Or one could say that they were experimenting, copying some humans that they saw kissing in the castle.