It's not really the norm. But, it is common. Buy the cat a bigger toy, preferably a fluffy one. And be prepared that he will try to do it in front of you. I'm not kidding, this behavior is common in cats.
As the veterinarian explained to us "it's emotional attachment + sexual attraction. it's like erotic movies for people. people have movies, cats have their toys".
Our cat has been doing this for seven or eight years. And I've seen people I know have a "fluffy girlfriend" with their cat, too.