I'd imagine that exceedingly few people enter into a marriage thinking that they're going to have an affair. To most newlyweds, the very thought is almost incomprehensible — I'd never do that. Not me. Not us. The truth is that it can happen to anyone; you are capable of cheating on your significant other, and I think acknowledging that is valuable in protecting your relationship. But affairs also require a sort of perfect storm of circumstances, and I think being mindful of those circumstances and understanding how to prevent or minimize them is an important strategy.
I'm a personal trainer and a gym owner. Over the years I've gotten to know many of my clients personally, many of them attractive females close to my age. And I work in an environment in which I regularly interact with very fit, tightly-clothed beautiful women. So maybe it's that, coupled with my year-long relationship with a married woman, that makes me especially conscientious about sexual attraction, emotional attraction, and the circumstances that can lead to infidelity.
1. Small things
The first circumstance is an erosion of marital intimacy. This doesn't necessarily happen as it's depicted in the movies, where there's a series of huge conflicts or fights in which hurtful things were said. Instead, it's the small things: not making quality time a priority, and spending it on friends, work, or hobbies instead; going for extended periods without physical intimacy — and I don't mean just sex, but kissing, gentle touching, holding hands, cuddling, etc.; and neglecting compliments and verbal expressions of love, like "thinking of you", or "you are incredibly sexy".
This erosion of intimacy comes as stress, jobs, and other obligations spur complacency with those physical and verbal expressions of affection. Simply being around your partner regularly and becoming accustomed to them is sometimes enough start taking them for granted, but the disconnect between partners is one that happens slowly, in small steps.
2. Physical attraction
The second circumstance is physical attraction between two people. Sorry, it's not enough to just think "that person's hawt!" We're human beings, and it'd be naive to think that marriage somehow inoculates us against finding other people sexually attractive. I grew up in an evangelical Christian community that preached Jesus' words: "Any man who looks at a woman lustfully has already committed adultery with her in his heart". Well, fuck that thought-crime nonsense. I've looked at tons of women "lustfully" and I don't feel an inkling of guilt about it. I fully expect my wife to cast her eye toward other men she finds physically attractive as well. It's just biology, folks. And frankly I would conjecture that, as is often the case in the religious community, repression and guilt simply magnifies those desires. If you can accept that you're a human being with a healthy sex drive and that you will find yourself occasionally admiring other humans, you can skip the guilt and go about your day.
Mutual physical attraction certainly propels relationships forward and can contribute to affairs, but I think it's the weakest factor of the bunch. For me, the fact that I occasionally find myself physically attracted to other women can't come close to outweighing the fact that not only am I very physically attracted to my wife, but our relationship is built on much, much more than that. The real risk with physical attraction comes when it's combined with emotional attraction.
3. Emotional attraction
In the time since I met my wife, I've met plenty of other attractive women; to some degree, as I mentioned above, it comes with my line of work. But in order to propel physical attraction into an affair, you need an emotional connection. And to some extent, I can confidently say that I've met a few other women who, were I single, I'd be interested in.
That was the case with my previous relationship. There was a physical attraction, sure, but what really drove the relationship was an emotional connection. We related to each other. We had similar interests and views, a similar sense of humor, and enjoyed getting to know one another. At no point were either of us (to my knowledge) pursuing an affair. Our relationship grew organically with the time we spent together. I was single and, after a nasty breakup, was resistant to jumping into a committed relationship; she was in circumstance  in her own marriage, and our mutual attraction grew with the time we spent together. And that is the fourth and last circumstance:
The above circumstances alone aren't enough to spur an affair; like any relationship, the relationship that will become the affair has to grow and be nurtured. Physical attraction lays the groundwork for emotional attraction to grow. And if you're dissatisfied with your partner and choosing to spend time elsewhere instead of working to rekindle your intimacy, the excitement of a new relationship can be enticing. You start looking for excuses to spend time with the other person instead of your partner, and in time the small escalations can culminate in an affair. All along the way, you may think, "Not me; I'd never cheat on my partner! I just really like spending time with this person. My partner is always busy with this and that..." and so forth.
I remember that my relationship with a married woman certainly didn't materialize overnight. We'd hang out with mutual friends, and talk after they left. We'd spend time with each other when her husband was busying himself with friends and hobbies. I felt a connection and an attraction, but I was in complete denial — She'd never do that. No way. They're both my friends.
Even after she kissed me softly on the cheek and held my hand, I chalked it up to an unlikely set of circumstances. A few weeks later, a passionate kiss. I still figured it wouldn't escalate, that she'd snap out of it even though by then I was falling for her. It wasn't until we slept together that I knew we were in deep. We managed to keep the relationship going for a bit over a year, until she became so consumed by guilt that she had to leave. I haven't spoken to her in years.
Caring for what you have
Looking at her own marriage — though of course I only knew what she told me — I knew that it wasn't harsh conflict that drove them apart. It was steady neglect. She felt that she wasn't important to him, that he'd rather spend time with his buddies than her. She told me once that she asked him to look her in the eyes and tell her he loved her; he couldn't.
I remember that while I was involved in that relationship, I read an interview in GQ with the woman with whom the politician John Edwards had an affair. She said something that rang very true:
... infidelity doesn't happen in healthy marriages. The break in the marriage happens before the infidelity.
I'm truly, madly, deeply in love with my wife. She's my best friend, my partner, my team-mate. I can't imagine my life without her. Our marriage is young, but I don't want to fool myself into thinking that we couldn't end up just like my (quasi) ex's marriage. With the divorce rate as it is, I honestly don't know if my wife and I will make it; I don't think anyone does. But I have to believe that if I can be diligent in nurturing our marriage and be mindful of the kinds of scenarios that make infidelity more likely, I can tilt the odds in our favor.