Firstly, I don't think compulsive behavior, even compulsive flirting, always needs to be a problem:
In general (not your specific case), I'd say compulsive seduction attempts fall into the category of players who have compulsive behavior that isn't the main focus of the game according to the GM or other players. In general, I think the other players and GM can make some effort to interact with the player's interest to see if it can be interesting or enjoying for all, and if/when not, it can be dealt with in game, and/or talked about OOC.
It doesn't seem that difficult or unreasonable to me in general to deal with such things, unless/except when the other players have some issue with responding to in appropriately. So when it is a problem, I think it's worth looking at why it's a problem for the other players, as well as what the player with the compulsion is looking for, what they think is interesting about it, and if there's any way to have that be resolved, reduced, and/or be interesting for others, too.
I do think that there are likely several issues in your specific case:
1. There may not be enough negative feedback from other characters to the bad behavior:
As for seduction attempts themselves, those can be interesting, but if (as in this case) a character is doing it compulsively and sloppily, then it seems to me the easiest and most natural way to handle it is the same as in the real world. I.e. it puts people off, both the targets and the people around them, and so those PCs and NPCs would tend to take escalating measures to shut it down. Any time a PC behaves badly, it seems to me all NPCs who witness it can and should react appropriately. Constant bad behavior is a serious character flaw which in a realistic game such as you want would quickly undermine his social standing, and the reputation of his associates (the other PCs), and would have significant effects until it's dealt with.
2. The problem PC may be a terrible, clueless flirter:
It sounds like this player may actually be clueless about how courtship works, and how getting along with others works, and if so he may need some lessons either OOC, or perhaps demonstrated in play. On the other hand, occasionally forward jerks find a receptive audience, but mostly they make everyone who's not like them annoyed with them. I think what tends to be the problem is when some RPG conventions may get in the way of corrective social consequences. That is, in real life, no one hangs around jerks except other jerks for very long unless they feel like they must. Ideally I'd suggest that you just have realistic social consequences - when he acts like a slimy fool, everyone stares and treats him like a slimy fool. If that can't be managed or fails somehow, then I'd suggest having an OOC discussion with him.
3. You and/or your leader GMNPC seem to not be giving effective negative feedback:
Also, what was it about the leader GMNPC that had her not able to shut him down or escalate when he persisted? The GMNPC leader with pious asexuality actually conceded to a series of annoying incompetent advances? Why? Did your character have a weakness around that, or do you? Seems to me it wouldn't take a lot of time if you gave no reaction or ignored him, or threatened him as the leader, especially if you followed up on threats. If unable to do any of those, your character could threaten to leave the group unless he ceased.
4. The other PCs may also have issues giving appropriate negative feedback:
Finally, is there a taboo against inter-party conflict? Seems to me that a PC bringing along a love interest and getting another PC killed because of it, would tend to lead to the other PC's taking action or at least insisting on a behavior change for the lecherous PC. Why aren't they doing that? I would consider what they have said and done, and not said and done, in the face of their comrade behaving horribly, messing with their leader, and getting one of their other comrades killed in a foolish way. Are they just roleplaying their characters? Or are the players clueless about what to do about inappropriate behavior by other PCs? Is that due to a meta-gaming agreement to always be a team no matter what, that somehow has a blind spot about the betrayal inherent in behaving terribly? I'd try to figure that out, and some suggestions, and then I'd talk to the other players OOC about it, and try to get them to agree on effective solutions.