Here's the conclusion:
Lopez: What's the most important practical advice from "Smart Sex?"
Morse: Love is worth the effort. Love is not a feeling: love is a decision to will and to do the good of the other person. And in the end, it is the greatest adventure of all.
Lopez: So what counts as Smart Sex?
Lopez: I take my motto from Sherlock Holmes, who once told Watson, "after you have eliminated the impossible, what remains, no matter how improbable, must be the truth." After eliminating all the forms of dumb sex, hooking up, cohabiting, divorce, and remarriage, what is left? Life-long married love. Although it seems improbable to the modern mind, the truth is that married couples have more and better sex, and have a far better track record at dodging the dumb sex that has caused so much misery.
Married sex is smart sex.