Does it count if my character and an NPC seduced each other simultaneously? Because seriously, I think I might've done that with a drow priestess. Started with the party getting captured, then she arranged to meet me (a fancy ranger variant) behind closed doors so she could hear me "beg for my life", and the DM mentioned that she had horns and unusually light skin for a drow: I rolled well on a knowledge check and identified her as a part-fiend of some kind, probably either a succubus or erinyes. So I started by telling her it was too bad she wasn't as good-hearted on the inside as she was beautiful on the outside, or something like that. Natural 20 Diplomacy. She paused, the DM rolled some dice behind the screen, presumably for a sense motive roll, and nodded.
She replied by cautiously asking if I really thought she was attractive, despite her obvious heritage. My DM also told be to roll opposed Diplomacy, and I did: an 18, good enough to keep my wits about me. So I replied that yes, she was "the hottest piece of ass in the whole gods-damned Underdark", but that I couldn't really love her because her heart was black. We rolled to change attitudes: we'd already gone to unfriendly from hostile, and again both rolled well. I dumped off a daily bonus, and got her all the way to helpful, and went from there.
I told her that the only thing keeping me from "whisking her away to the surface to be my blushing bride" was that she was keeping my party here, and the DM, who had no qualms about hamming it up in front of the table, told me "not to make a promise to a girl if I don't intend to keep it", and that if I swore to be hers she would even "let me take the reigns every once in a while".
They went in ready to kill each other, but by the time the party got there, they were cuddling and sipping mushroom wine.