Well, it is canon that the woman who gave Kim The Disease was "different" down there. On the other hand, this wasn't said about Kes. As for Seven's offer, they were in the mess hall, so it is absurd to think that this was a serious offer. Seven was calling him out for getting horny for her. The real issue is why it was supposed to be so outrageous for Kim to aspire to Seven. In pessimistic moods, it seems likely to be an issue of relative coloration (his wrong for hers.) In more optimistic moods, it seems to be an issue of metaphorical incest, between Janeway's "son" and "daughter."
I think the issue has to be resolved by Favorite Son. Viewers were repulsed in droves by this. I'm not quite sure why anyone wouldn't be amused by the vaginal doorways. But Harry Kim was quite tempted. Straight.
I'm watching/rewatching Enterprise on Netflix and it is quite striking how similar Trip and Harry Kim are, personality wise.