Any two people can have a happy relationship. The only absolute essential is that each partner put the needs of the other first. For men that usually means ego gratification (through sex) and for women it usually means financial security and romance.
If Adolph Hitler had married Mother Theresa and they each put the other on a pedestal and the needs of the other ahead of all else, everything would have fallen into place. Adolph could have become docile and harmless so as to not worry his wife and Theresa could have learned to do what Adolph likes in bed. Too bad Eva Braun was not Mother Theresa.
Actually, I’ve noticed that the most successful marriages are those where both parties are each other’s best friend.
When a couple suddenly experience sexual problems, if they are otherwise in a close relationship, a search for a solution to whatever caused the problem will be sought without either adventuring into someone else’s bed.
Marriages based solely on “great sex” do not endure.