The key to a happy marriage is to not go to bed angry.

I disagree.  The key is to talk about why you are upset.  And, if it cannot be resolved by bedtime, you WILL go to bed angry.  However, it will be with the FAITH that you can revisit the anger in the morning.
Anger is like any other emotion.  It’s natural.  It’s expressive.  And, EVERYBODY experiences it.  Like all other emotions, sometimes you can’t “control” feeling or not feeling it.  And, again, like all other emotions, its intensity is not always easy to process in 24 hours.
So, why is the key to a happy couple to not allow yourself to feel THIS emotion?
Because people are afraid of anger.  Anger is associated with violence, aggression and rage.  HOWEVER, that is not the feeling, but the manipulative, unhealthy, and inappropriate expression of the emotion of anger.  If the phrase was truly referring to going to bed aggressive and being violent, I would 100% agree (but who falls asleep being aggressive and violent…SMH).
A healthy marriage is when people can go to bed owning their emotions and experience the freedom to feel them and the safety and openness to communicate them.  It’s not healthy to internalize anger, deny anger, or dismiss it, if what has angered you has not been resolved.
So bump the myth!  Be yourself and own who you are and feel what you feel.  That is one of MANY keys to a healthy marriage.  IMHO