I just read "Vow," by Wendy Plump, in which she writes about her own and her ex-husband's infidelity; the benefits and the costs she found.
To have a marriage of fidelity, you have to employ loyalty, to stay out of potentially dangerous situations, to keep putting more into our marriage, to keep the windows and doors shut as Mira Kirshenbaum writes in her book, When Good People Have Affairs: Inside the Hearts & Minds of People in Two Relationships. And by all means, know that alcohol is a hugely dangerous inflammatory bomb.
As Stephen wrote on Couple's Net, he remembers how lucky he is to be married to Nancy, and presumably, acts within and without her presence, from that place of knowing just how lucky he is.
Plump writes about losing the passion in her marriage, about the allure of the newness of falling in love. You can fall in love with your mate, every day. You can look, see, listen and know her, every day. You can touch, affirm, and give to him, every day. It is a choice.
You can get lost in the daily rhythms of groceries, kids, work, laundry (oh yes, and devices). Or you can be in it together.
It's tricky because the hormones and chemicals that get going when you are allured are real hormones and chemicals flowing through your brain and body. They always subside. But they are addictive, as are alcohol, drugs, and other behaviors that trigger the reward system in your brain.
Mature love is different, and with effort and loving care, passionate and comforting, both.
Fidelity and loyalty happen in the place of conscious competence (from the Four Stages of Competency: unconscious incompetence, conscious incompetence, conscious competence, and unconscious competence). You have to pay attention to what (and who) is around you, and choose your partner--again, today.
The human brain is wired for complication and leans toward negativity. It is up to you to slow down and get your left frontal cortex on line so you can make informed decisions. Most people really do want to be happily married. Most people really do not want to blow up their family.
Be sure you spend time with people who are friends of your marriage. It's easy to hang out with those who will vicariously live the thrill of an affair through you while s/he goes home to a safe, loving home.
Keep talking with your spouse about your day, your week, your life. If you find yourself talking to someone else about things you are not talking to your mate about, or are hiding who you're talking (or drinking) with, stop now, and go home and put more into your relationship.
Monogamy ultimately is a commitment to one's self. Loyalty and integrity are what you do when no one is watching or will know.