Part of the reason Mona left was because she didn't want to bang old guys.
She will 100% have old guys leering at her, feeling her up, paying for VIP treatment... not to mention fat guys, ugly guys, any other kind of guy that turns her off, will be a customer. She can't afford to be picky in a job like that, it won't always be private DIK parties where every guy is young.
Sarah and Mel already fuck around a lot, they probably see the restaurant as easy money, rather than something they need to do for the money. But then again, we don't really know most of the girls' situations.