You could always use the chat option of "NO" and stop the romance right then and there. Just like every other RPG out their with dialogue choices where you can stop the romance before it starts.
You shouldn't have to fend off "weird sex cult" companion advances. For me personally, if a companion randomly says why he/she wants or doesn't want to bed my PC, it already sours the party dynamics/companion interaction. There wouldn't be a problem if romance was 100% player-initiated. (Or preferably if there was a toggle to block all romantic interactions in the party.)
