I might be reading too far into it, but is heavy another song about cheating? I mean it is clearly about a one night stand but something makes me feel like it's one that happened during a relationship. It's mainly based on the second verse:
"Maybe in the future, there might be a future, right now there's no future, it's fucking useless"
Why is there no future right now, it's not because she doesn't like him, since the possibility exists in the future. There is no future because this was cheating, she's already commited to someone else.
"secret, heavy, dangerous, international, one night, deadly"
If it's about cheating these words get a whole new meaning. In fact for a regular one night stand I wouldn't use words like deadly or dangerous at all.