Probably because homophobia is discriminatory. That and the rules state "...ALL kinds of discrimination, including but not limited to racism, sexism, anti-furry, (especially) homophobia, and more. Please report any violations to a mod. Sharing stories involving this is fine."
So yeah, while I don't have the full story, it seems to me that your friend was "discriminated" against for discriminating against others. They went against the rules, so they got banned.