I really feel for you, I've had this problem before, but I think you should cut yourself some slack, who feels like sex when they're feeling ill?
I think libido is mostly in the mind. For me, ill-health makes me not want to have sex, which makes me feel bad because I feel like I'm letting my boyfriend down, which makes me feel pressured into wanting to have sex, which makes me stressed and makes me not want to have sex even more! The pressure of wanting to start a family must make things even harder. Kinda takes the fun out of it doesn't it. Maybe if you try and dedicate some time for the bedroom, and try and view sex as something you can enjoy regardless of crohn's, rather than as something you should be doing to make a baby, or to prove you can/want to. If you can relax and just have some fun with it I reckon you'll feel better in no time.
Either that or you could try eating asparagus, in a candle-lit bubble bath, listening to romantic music - that's gotta work! Good luck and I hope you and your wife get pregnant very soon