No, not all men, of course. But the majority of men do cheat on their partners and that is a sad fact. Almost 20% of divorces are on the grounds of marital infidelity, quite a high proportion given all the other reasons for a marriage to end. A recent research study by Oxford University found that "While percentages of extramarital sex (EMS) vary from study to study, it can be estimated that 50-60% of married men and 45-55% of married women engage in extramarital sex at some time or another during their marriage." Among gay men that figure may well be higher.
Sure, but infidelity is probably more likely a symptom of a failing or failed marriage rather that the cause of one. And, don't forget, in places where "no fault divorce" is not a legal option, couples have for decades agreed that one member (usually the male) will "admit" to infidelity as a means to end a marriage even where no infidelity has actually occurred.
I've been married 34 years, have never cheated on my wife, and am confident that she has never cheated either. She has engaged in some "hotwife" activities both solo and in MFM threesomes with me one of the M's, but that was mutually agreed upon activity (indeed, at my urging) and I do not consider it cheating.