Romance is the key.
There are times when the act of intercourse is all that one or both of you have left (strength-wise, etc.) with life's demands. But we must work hard not to allow that to become the routine. Because then it will devolve further to deprivation: masturbation (which is not the second unpardonable sin).
In the Bible passage used to preach against masturbation, Onan did not even touch. He had actual intercourse with his sister-in-law Tamar (according to levirite marriage practices to produce heirs for his dead brother), but he spilled his seed on the ground to avoid getting her pregnant. So the sin God judged him for was his turning a levirite marriage into casual sex. (Genesis 38:2-10)
There are times when masturbation is a necessary thing in marriage. Married people deployed in the military for months and years at a time. Menstruation monthly and after child birth.
That point in bringing this up is that these are matters of the last resort, both of which can be ruts married couples can fall into which take the place of good healthy sex.
Romance is part of sex. The Song of Solomon is a good read filled with romance. The loving nudges, the flowers, the poems, the terms of endearment, the exclusivity, the naughtiness.
Dr. Dobson once wrote (paraphrasing) "When told by a patient that he had not had sex in a while in his marriage, I asked him if he helped her around the house?" In other words, helping with the dishes can be romantic. On another occasion a woman with the same complaint was advised (by whom I do not recall but it was good advice) "Consider greeting your husband at the door with nothing more than earrings on.
It's these things that keep the fires of passion burning... that keep each other's attention exclusively on each other... that keep the love life alive.
The statistics bear out sadly that Christian divorce is as high or higher than non-Christian divorce. And 99.99 percent of Christian divorce is based on infidelity. I know we are all only human, but we have the Spirit of God living inside us (who constantly tries to teach us and to warn us if we'd listen). So, I don't believe Christians set out to have an affair.
It begins subtly, one degree at a time. For example, John and Jane Doe decide to live in a high-dollar tax bracket / life style. It requires John to work often into the wee hours, and for Jane to have a job outside of the home. When they do see each other at home, they succomb to having "quickies" to satisfy their urges. There's no time or energy left for much else.
As time goes on, masturbation is an even quicker "quickie." And the two are little more than roomates. Both having needs and neither getting them fulfilled by the other. Be certain, there are others out in the workplace or marketplace or leisure place who are ready willing and able to fill those needs. Quite often they are married people as well not getting their needs met.
The glances, the meeting of the eyes, the smiles, the flirting, all these things that ought to be part of the romantic build up to sex in a marriage start being expressed outside of marriage. And sex is too powerful a temptation to be dealt with by the flesh. The Bible says the men undone by the harlot were POWERFUL men. It also says to FLEE sexual temptation.
Most caught up in it give into it and adultery results (actually adultery is prosecuted because adultery acording to the Bible began when romance was aimed at anybody other than a spouse).
This is not to pile a burden on weary people (men or women). It is to be sober about what we are up against and the subtle compromises we all make... and their ultimate consequences.
What could the Does have done differently?
Changed their life style and income expectations to allow them the time and the energy to pursue a healthy sex life in their marriage. It really makes no sense not to consider this in that it will eventually lead to the destruction and end of that very marriage in nearly all cases.
A better question in this thread might be "How many quality (romantic) times do you have sex?
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