I disagree fundamentally with this assessment. Player A is deciding the emotional context for his roleplaying regarding player B's character.
That's splitting hairs pretty finely. Considering that, presuming Player A is a good roleplayer, that decision will affect every single interaction these two characters have, to the point where Player A will base decisions on this relationship (which was brought up in the original example), it's not like this is happening in a vacuum.
Now, if it happens 100% in Player A's head and never comes to the table, well, sure, then Player B obviously can never complain because well, Player B will never know what's going on.
But, since Player A's actions are predicated upon the presumption of a love interest, even if it's not reciprocated, Player B is forced into this situation. Player A's character's behaviour towards Player B's character is entirely the issue here. So, yes, Player A's character is most certainly doing something to Player B's character.
Look how you phrased that. Player A decides his emotional context for his character's relationship with Player B's character. That's the whole point. Player A unilaterally decided this, which does impact Player B's game without any consultation with Player B.
He did so in the thought that this would make for an interesting game.
He is mistaken. It might be more interesting for him, although, I strongly doubt it. Knowing that his behavior is making the other player uncomfortable and knowing that his behavior is ruining someone else's game, how could he honestly claim that it's making the game better and more interesting?
And to me, that's the whole point. The basic purpose of introducing this was to make the game more fun. Sure, no problems. But, it failed. It did not make the game more fun because it outright ruins someone else's good time.
Anyone who would continue to enjoy their behavior at the expense of others isn't someone I really want at my table. As soon as I know that my behavior is ruining someone else's fun, why wouldn't I stop? Sure, I might be having fun, but, fun isn't a zero sum game. At least it should never be. Knowing that my fun is ruining someone else's fun is going to make my fun, well, unfun.
Am I wrong for thinking that?