The way I see it, if there are two or more characters, and they are engaging in some sort of physical contact (or physical contact by proxy - such as a sex toy) which is able to, at least theoretically, result in an orgasm for at least one of them, then it's sex.

Note that the key here is that the physical contact should be enough to potentially result in an orgasm, so even though someone might get off from a deep kiss, that's a mental thing and not a physical thing.