Yea I prefer on top, I think I would reluctantly go under the covers with them but I've only had one ever ask and by complete coincidence that day I'd just thrown the duvet and a throw blanket over a sheetless bed because i was in a hurry, that was embarrassing!
But yea if it makes you uncomfortable don't do it, there will always be more potential clients but if you start reneging on your boundaries you might feel unhappy in the long term.