The pat answer to this Nikki is that most of the time, the men you describe are cheating with a woman and you acknowledge yourself that women also have affairs, with other men and other women, but I think the social conditioning argument is more interesting.
I think the gender politics at the moment are fascinating. I have noticed that there is still an expectation amongst men (and OW) that men would be mad to turn down the opportunity of extra-relational sex, at a time when more women are considering and offering no-strings attached sexual relationships.
Since we are in an era of increased technology and mobile phones, emails, messaging and social networking sites feature so heavily in modern affairs, these act as an accelerant to infidelity, but are also the most significant tools of detection too.
What I also see causing a new crisis of fidelity is that despite the realities of people having opposite-sex friendships, working closely with colleagues away from home, contacting old friends on FB and FR, couples blindly assume that they are invulnerable to temptation and consequently never discuss fidelity and the challenges that will happen in most of our lives, when we develop a crush on someone else that is reciprocated.
I think the biggest threat to fidelity is the under-estimation of how difficult it can be to resist temptation and this applies equally to men and women. However, to progress this, it makes sense to look at the opportunities for infidelity and where they are most likely to occur. Societally, since we still pursue a model of men working and women child-caring, proportionately more men are at work where opportunities are greater.
I believe that if we ever saw a society where women felt a pressure not to turn down the opportunity of free sex and where child-care was undertaken by mainly men, while women worked, the gendered disposition to infidelity would also change. Other things would change too, such as the current greater societal distaste for a woman who leaves her H and DCs for an OM, or the assumption that even if a woman is unfaithful and leaves her H, she will retain custody of the children and her H loses not just his wife, but daily contact with his DCs.
I'd rather see a society where relationships were more egalitarian and where there were no expectations that one gender prefers sex more than the other and where there was a cessation of the bargains we see on here all the time, whether it relates to porn consumption, infidelity or any other marriage-reducing behaviour - the ones that start from the premise that it is only to be expected, because the people doing it are men.