The dictionary defines sex as "Sexual interaction, usually involving vaginal and/or anal and/or oral penetration".

e621 doesn't have a different definition, so this might be a good one to go by.

Clawdragons said:
someone might get off from a deep kiss, that's a mental thing and not a physical thing.

Whether or not kissing is a psychological (i.e. mental) thing, it is definitely a physical thing, because it is a physical act that involves physical touching.