If pc B gets creeped out, the way to handle it is in character. If player B gets creeped out, it's worth talking about the situation; but really, telling player A he has to compromise his pc's emotional attitude is like telling someone that they can't play a halfling because you're uncomfortable with little people, even though you're another player instead of the dm.
When you go to the OOC appeal to a player because you are uncomfortable, there should be an understanding that you are taking an extraordinary step.
Ok, I think I've twigged to one of the two big areas of disagreement here - it looks like some prefer that this sort of thing should always be handled in character.
For me, I think it is great when such a conflict can be handled in character, and it can even offer some really powerful roleplaying opportunities.
But I think there are also times when OOC is not just acceptable, but is even the more appropriate response. When we're dealing with an issue that is genuinely outside of another player's comfort zone, especially when it is intruding on that player's character directly, and they aren't comfortable playing through
any ramifications of it - it just isn't an experience they want as part of their game. I think that's the time when the players need to sit down and talk it out.
You note that its ok for a group to discuss and determine what is acceptable before a campaign, but that once it starts, that isn't acceptable. I simply... don't see why. It just doesn't seem reasonable to say that because a player didn't specifically mention they don't want other characters romantically pursuing them, they have to accept it and try to compromise with the player initiating such an uncomfortable situation. If a situation comes up in game which crosses their boundaries, they should have the opportunity to speak out and ask that it stops.
Now, from there you say that Player 1 shouldn't have to curtail their roleplaying experience in order to satisfy the needs of Player 2, and thus both sides should be equally willing to compromise. I'm not sure that is true, either - as some have mentioned, that seems awfully close to victim blaming. Player 2 is in some way 'at fault' for being offended or made uncomfortable by Player 1's behavior. Player 1's roleplaying experience has equal weight as the personal comfort of Player 2.
Which brings us, I think, to the second big point of disagreement - when a boundary line is truly crossed.
Like the Jester notes, some demands might be unreasonable ones. If I'm sensitive about my height, is it fair for me to insist that no one is allowed to play halflings?
On the other hand, Celebrim seems, to me, to be setting the boundary line
waaaay past what I think is appropriate: "it requires a lot more unconsensual and extreme case than anything that has been outlined here. An example would be essentially character rape, where one player dominated another player and forced them to engage in sexual acts or other perversions against their will".
I think there are plenty of reasons for someone to be uncomfortable with the game content
way before it gets to anything this extreme.
For me, a big part of the line is when another player's roleplaying is involving my character without my consent. Especially when getting into something like romantically pursuing a character which, yeah, can absolutely make people uncomfortable with unwanted attention. And I think someone's discomfort over that sort of behavior does take precedence over how someone else wants to play their character.
Respecting the boundaries of other player's should be one of the agreements of the game. Now, if one of the goals of the game is specifically to push those boundaries, and the DM has said up-front that this is the case - or if, when conflict arises, all the other players say they are fine with that sort of behavior - then the person being discomforted should probably be the one to leave. But in most games, that isn't the case, and I don't think "not having to deal with harassment" is an unreasonable assumption for most people going into a game.
From what I can tell, some of the folks here, like the Jester and Celebrim, seem to be arguing that the roleplaying purity of the game should take precedence over the emotional boundaries and personal comfort levels of the people playing the game. Which might work for your groups, and fair enough.
But for me, the enjoyment and comfort of the players comes way before the 'roleplaying purity' of the game.