Do you at least recognize that player 1 might have his fun spoiled by having player 2 tell him that his pc's motivations need to be re-written?
Sure - as I've said several times already, it's possible that this might reduce the fun for Player 1. But I also feel that the loss of fun for him is likely to be significantly less than the loss of fun for Player 2 if the unpleasant situation continues. And if that isn't the case - if Player 1's fun actually requires forcing an uncomfortable situation on another player - than there may be more serious issues at play.
The difference is that Player 1 should have countless other options available that they find entertaining. If they want their character to be motivated by some sort of romance, they should be able to talk to the DM and see if they can come up with an NPC for the role, or see if any other PCs are interested.
Going back to the original scenario. If Player 2 lets things happens, it means they get to sit through the rest of the session watching as another player's character performs actions, directed at their character, that feel like stalking. This is an unpleasant experience and ruins the rest of the sesion for them.
If Player 1 backs down, instead, it means... they find a different romance to pursue. I just can't see any scenario in which that holds the same emotional value for them as the consequences for Player 2. If they must pursue Player 2's character in order to enjoy the game... again, it seems like there may be a much bigger problem going on.
I guess I fall firmly on the "Game is more important than any one player" side of things- or perhaps a more accurate way of putting this is "Each player should find the right table for him or her comfort zone, but should NOT try to change an established table's social contract to suit themselves."
And if it was declared to someone at the start of a campaign that being unwilling pursued by other PCs was part of the game - or if a player objected, and all the other players agreed that this was a central part of the experience for them - than I agree, that's a situation where Player 2 should back out. (As I acknowledged very early on.)
I mean... again, I'm probably approaching this from a different angle than you guys. I game primarily to play with my friends. Say a friend in my game says to me, "Just so you know, I have a severe fear of spiders... big human-sized ones don't bug me because they are obviously unreal, but I don't think I could handle dealing with small, genuine skittering ones."
You know what I would do? I'd avoid throwing swarms of tiny skittering spiders at him! There are so many other things I can use in this game that the loss of one element is easy enough to handle. The Game being more important than the Player is simply a self-defeating concept for me - the entire point of the Game is to have fun with my friends in the first place!
That said... sure, there are limits. And they will change from one group to the next. And when you start getting into broader issues, you may well have no solution available - if one player insists on only playing in monotheistic games and another insists on only playing with the standard D&D pantheon.
But when it comes to a player's opinion over their own character? I can probably avoid having spiders crawl over them in their sleep, or have them be unwilling pursued when it comes to romance, or force them choosing between two morally-unacceptable decisions (sell your soul to the devil or this child dies!) If I know those will be issues for them.
Remember, we aren't saying anyone needs to avoid plot elements because they might offend someone. Just that, once you know something is a problem for the player, if that element can be removed without compromising the integrity of the campaign... I, at least, will generally find that a far better choice than losing a player entirely.