Far as I can see, Blizz really isn't into heroes having relationships with other heroes outside of Pharah and Ana's family connection. Some are buddies, but generally those with romantic connections are paired with civilians. As for Genji and Mercy, it's not like he doesn't have other reasons to give her chocolates.
I am amused that a lot of the "Tracer kissing Emily doesn't mean she's queer" crowd are totally on board with "Chocolates mean destined romance".