This might just be different customs of language usage. But to me, if you say "This is a subset of That", you are saying "every case of This is also a case of That". I suppose you are using it to mean "That does not imply This", and honestly I don't think seen it used that way elsewhere.
I mean, that is the immediate conclusion one can draw from what you're saying. It sounds like you're saying all sexually exclusive relationships are within the group of healthy relationships, which is different than saying only some of them are within that group.
