You want to hate them. Don't.
The problem in this situation seems pretty obvious at first glance. Your ex is gay, and you are not. That is just the dash of rainbow sprinkles on top of the breakup ice cream cone. After all, if you had known they weren't into your assigned gender, you probably wouldn't be in this situation in the first place.
The actual problem, to be blunt, was this: your relationship was based on a pretense. You have every right to be upset, even irate, about this. But before you curse them out or call names, stop and think for a minute. Ask yourself that one simple question: why would someone lie to you about his or her sexual orientation?
Generally, the answer boils down to one of two possibilities. One, they were genuinely unsure of their preferences; or two, he or she felt the need for "straight" camouflage. In the former, the lie was unintentional. There’s not much either of you could have done to avoid the outcome. And to a certain extent, this person liked you enough as a human being to find out if things would work... and actually, that's the whole point of dating.