It’s touching a highly sensitive part of your anatomy to part of another being. This proves that you trust them.

The touch is gentle. This proves that you are to be trusted.

Both contracts are those of mutual safety.

There are multiple types of kissing. There’s the kiss, perhaps on the head, that you might give a pet or a young relative.

And there the other sorts of kisses that would be incredibly inappropriate in those instances because it breaks part of the contract of trust.

But if between consenting individuals who trust each other in other ways, then all is well.