ELI5: Why do many gay people take it harder when their partner leaves them for a member of the opposite sex as opposed to the same sex?

Of course it isn't ALL gays, but some.

I was watching the show Catfish and the girl emphasized that her girlfriend left her for a man. I realized I've seen this before and just wonder why it's more hurtful to be left for the opposite sex, rather than just left in general.