Rules of all kinds should always be taken in context and never out of it, as it gives us a clearly image of why that rule exists. Mixer has decided to make a rule like this because of the goals they have for the service, namely to be a platform for every one regardless of age. They don't cater to every kind of audience, people seeking some nice soft porn being an audience they don't even want to focus on. There is nothing wrong with this and this isn't discrimination as there is a clear and logical reason to have these rules other than "I don't like titties or girls".
Say you have a straight bar, if you were to ban black people then you would have a problem as there is no other logical reason in doing so other than you being a racist about it. Then take a gay bar where you ban any and all girls from entry. That isn't discrimination, that is logical within the context of the rule as girls would actively ruin the purpose of the bar. Mixer does the same thing currently. They want a platform for all ages and having girls quite literally selling their bodies to gain viewers is something that actively ruin the purpose of the service within today's society.
You could argue that society is messed up for not allowing both men and women to go around naked if they so please, and you'd be right to some regard. Though then I'd raise a counter argument in this case that Mixer wants to focusing on streaming and more specifically the streaming of games and that they do not want to focus on soft porn for their platform. Meaning that within the context they are still well within their right to refuse certain elements without it being "discrimination".