Bottom line is this isn't about what any of us think is right, either musically or ethically. The agent says mixed bands are a harder sell for them, so that's it. Personally I'd disagree, but they know how they can make money.
'Entertainment' job ads are not ruled as tightly as ads for regular jobs - where you can't discriminate on age, race, gender, etc - the loophole being the person has to be right for a playing a particular part. It's pretty wooly but somehow how it exists without too much trouble.