Man I'd be creeped the hell out. That does look like a pretty planned out heart rather then an accident. Now if he knows your straight then hitting on you is sorta I dunno weird. I'd continue going to the burger joint and if you get another heart shaped burger talk to the manager. No reason to hurt this guys feelings by coming to him directly and confronting him. Confronting him would really screw you over. It would be a dramatic mess so I wouldn't do that.

Thats my best advice, ignore it and keep on doing what you do. If it re-occurs talk to the manager don't confront the employee. Also maybe bring your girl friend around again, she doesn't have to eat anything but him seeing your still together may keep him from repeating this obvious flirtation.