It's because Rio, Sarah, and Mel have been on the menu for the longest, it is therefore extremely fucking likely (like it's stupid to pretend that it's not what's going on) that they are a part of the free tuition deal, which means that they've slept with Burke, why would Burke give out free tuition to girls in Quinn's restaurant if he himself doesn't get anything out of it, doesn't make sense, Mona was more willing to jump into the deal head first, Cammy even says something to that effect I believe, that she only agreed to use the glory holes. So Camilla probably had sex with Burke through a glory hole at some other time, no way of know when or where, we never see that, but why would you assume that it hasn't happened, it's perfectly obvious that it was going to happen at some point.