Is it normal for husbands to request a threesome?

Recently my Husband of 27 years requested that we find a young guy to join us in bed for a threesome. We have been married for 27 years and always had a good sex life. He now wants to watch me have sex with a young guy and may join in if he likes what he sees. I have always worked hard to please my guy in bed but this feels out of bounds. Is this a normal request?