While none are justifiable all of these can be reasons why a man chooses to stray and cheat on his current partner.
Although genetics may factor into why men cheat on their partner it may also just be a week defense for a man who is caught in his unfaithful actions. A theory exists that testosterone levels in men make them more susceptible to infidelity. This theory is based on the hypothesis that men are genetically predisposed to cheating because they are ingrained with the notion that the survival of the species is dependent on their procreation. While this may be true it is certainly not an excuse for unfaithfulness in our age of overpopulation. It may be true that in prehistoric times, men were tasked with populating the earth this is certainly not true today and is not a defensible excuse for cheating on your partner. The truth is that in a truly loving relationship a man should be able to control himself and remain faithful despite any hormonal urges to produce offspring. The theory of man being predisposed to cheating does not hold water because while it may be true, there is no excuse for not being able to quell these urges.
Another reason men cheat is because they feel a sense of challenge in their infidelity. Actually being able to get away with an affair is a challenge that men enjoy. Additionally they enjoy the challenge of finding other women with which to cheat. To them the affair is more about the conquest then anything else. Sure they may enjoy the physicality of the affair, but the pursuit and sneaking around is what really drives men who cheat on their partners. They see fooling their partner as a tremendous accomplishment and they enjoy the logistics of orchestrating the affair without their partner learning of their transgressions. To them the thrill of sneaking around and the fear of being caught creates and adrenaline rush that is more important than the affair or even their current relationship. Men also cheat because they are driven by competition. If there friends are single or cheating on their partners and often have affairs with several women the man may feel that he needs to keep up with his friends. To these men affairs become a way of competing with their friends. Infidelity in men is often driven by a sense of competition.
Many men also engage in affairs because doing so is an ego boost for them. The knowledge that they can find multiple women willing to engage in an intimate relationship with them is a tremendous self esteem builder for some men. They take pride in the ability to attract women and don't care about the fact that doing so may destroy their relationship. To these men the excitement of the chase is almost more important than the conquest. Sometimes just knowing that other women find them desirable, is enough to stroke the ego of these men. In general it is a lack of self esteem on their part that drives them to seek out affairs in order to reiterate their attractiveness and desirability.
A man who is in an undesirable relationship may also be driven to cheat on their partner. If the relationship has begun to wane and the man no longer feels the same attraction to his partner he may seek out an affair out of boredom. If he feels his current relationship is lacking excitement he may also be driven to cheat in the hopes that he will find the excitement he desires elsewhere. While the wise thing may be to end the deteriorating relationship in search of a new more desirable partner, many men opt to hold onto the current relationship and engage in affairs. This may be because they fear that they may not find what they are looking for and may be left lonely and without a partner so they hold onto the failing relationship figuring that a bad relationship is better than none at all.
Men cheat for a variety of reasons. While none of these reasons may be justified, they do exist. The reasons that men cheat include genetics, competition, self esteem and boredom. In general while genetics may play a small part in the urge for men to cheat the majority of cheating takes place because men cannot control their sexual and competitive urges, are lacking in self esteem or are unhappy in their current relationship.