I was in a similar boat before. I was in the midst of cancer treatment and the like while I was dating a man who I really believed to be the one or close to it. Unfortunately, I had to have an emergency ostomy and told him so. As soon as he heard that, he said we should hold off until I get better. Obviously, if I do ever end up not having an ostomy, I will not be calling him. His nickname for me became "The Bag Lady".
Yes, Shannen Dougherty does have Crohn's; however, she is a really poor role model for those with Crohn's. She has been quoted as saying (not verbatim), "I refuse to talk about my Crohn's. It is embarassing. Women should not have problems going to the bathroom nor should they be talking about it. It is not sexy at all." I do know that Frank Fritz from the History Channel does have Crohn's. There are other people in history known to have it as well, such as Napoleon.
In telling someone you trust, you need to be relaxed and in a good atmosphere. I would suggest a playful approach, such as, "Ok. Coming right out of the gates. Tell me one of your deep dark secrets and I'll tell you one of mine." I think I would try something like that, but it really depends on your personality.