Ummmm....Hate to break it to you, but unless you have incredibly naive 17 and 20 year olds, they will not blush at sex jokes. I mean at 17 you are allowed to see an R-rated movie by yourself, so I doubt that any thing in one of these shows will disturb them too much. You won't see any full body nudity or anything if that is what you are wondering. It is all more humorous than explicit.
I just took a look at their upcoming schedule and I don't know when you plan to go, but the current show playing through the beginning of Jan. called Sugar Babies (never actually heard of this...) seems to be the most outrageous thing they have all year. The next show, "The Sunshine Boys" looks like it might be very funny. Everything else (Fiddler on the Roof, Kiss Me Kate, My Fair Lady, The Music Man....) all look pretty tame.
Also you can always look at their website for the menu for the currently playing show.