We could use the "if you're only Lesbian or Gay then you are discriminating" line. Bi would need to be the default here or they're discriminating against one sex or the other. ...or by their logic the infinite other number of genders out there.

Also, get a very long list of gender pronouns you can put down if/when your job starts forcing that in emails. If they try to call you out on it then just say they're being bigoted and discriminating against you and that they need to say the entire thing like they would with he/him.