Update:
I went into another save with a brand new sim and tried to replicate this and could not. One thing to note, this new sim is a non-occult and the sim I was playing with is a spellcaster.
I went back into my other save and changed his romantic preferences back to men. Then went back in game and had him ask to be just friends. He carried on conversation with Clement Frost and Care Ren without rebuilding a romantic relationship with Care Ren.
Having a hunch about the trigger, I had my sim ask Care Ren for fairy dust and upon receiving the fairy dust, that caused them to develop a romantic relationship, changing his romantic preference from just men to men & women.
My game is repairing now and I will try and recreate this issue again. I did record it happening, but there's no way to upload a video to the forum.
