Therapists, coaches, and books that focus on relationships have long said that adding novelty is one of the key ways to keep a relationship hot and happy. Now there’s even more science backing that up. “Sexually successful couples’ [are] more likely to have experimented with sexual variety—from anal stimulation to acting out sexual fantasies to using a sex toy together. And they had sex and orgasms more frequently. “It was encouraging to learn that more…