Alternate love interests that end up "losing" (some may consider it winning depending on the series) get no romantic closure of their own. Or even worse, they just disappear into oblivion.
Also... characters disappearing into oblivion in general just annoy the crap out of me. I remember Akira Toriyama would entirely forget about the existence of entire characters.
And of course, I've posted this before, but really it's not okay to abuse a character just because they are a guy and/or a loser.