Why can't you be gay and in a wheelchair?
Because you can't be a fruit and a vegetable at the same time.
Listen up, nimrod! This joke is saying that you can't be gay (a "fruit") and in a wheelchair (a "vegetable") at the same time. It's a play on words, using the terms "fruit" and "vegetable" as slang. You probably don't know what slang is, and I am not going to explain. Why don't you get back to your coloring book, idiot?