It is normal in that is happens to many young men. It may not be “normal” as in happening to most men your age. However, it is common enough that you certainly know other young men who have the same feelings for their mother, whether you know about that or not. It isn't exactly the kind of thing a lot of people tell other people they know.
There is nothing wrong with having these thoughts and feelings as long as they don't interfere with your reasonable obligations (such as your education or employment).
I actually know mothers and adult sons who've acted on such feelings, and some of them have the most beautiful relationships. It really doesn't matter if someone else is disgusted, and no, it isn't illegal in all places.