I´m not sure why the MC don´t tell about the restaurant to Sage. She´s the president of the HOTs and, if the "restaurant" comes to light, Sage´s good name and the other HOTs that are not into the bussiness is going to be affected too.
I think that, if the MC is not a "costumer of the restaurant", the dialogue between Quinn and the MC when she makes her offer and the MC rejects it should have been something like this one:
"- MC: This is another of your mental games, isn´t it, Quinn? If I say I´m interested you will make fun of me calling me pervert. Well, I don´t pay for sex, so I´m not interested."
"- Quinn: OK, yes, you got me, it´s a joke. Now out of my sight, pervert."
That way, the MC would not mention that to Sage or others because he believes it´s just a joke of her, or at least he´s not sure if the restaurant is real or not.
If the MC is a costumer of the restaurant, as you say, he don´t talk because he´s enjoying the situation.
About Sage knowing about the restaurant, that has been debated in other comments... why? She doesn´t get money from that business, she don´t need it, and I don´t think she would be happy knowing that her own foster father is a client. And she would know if she were into the ring. She just don´t know because her mind has been absorbed by her problems with Chad and her studies for her graduation, and has left most of the business of the HOTs in Quinn´s hands.