I wish we did know what causes it. But I don't think they're any closer to finding that answer than they are to finding a cure.
It can affect other parts of the body. It can break out inside the mouth (this is not fun at all! It's happened to me twice so far), it can affect the skin, eyesight, joints, spine........that's all I can recall right now. I've not had it affect my skin yet, but my eyesight, joints, and spine are all now affected. But please keep in mind that I have the worst form of the disease. I don't want to scare you and have you thinking your bf will get all these other problems. I truly hope he doesn't. Most people never do.
And yep, Crohnnies can have kids! I have 2 of the most wonderful kids on the planet to prove it..lol. So no worries there.