Large events seldom break marriages. Rather it is the little everyday stuff that makes or breaks relationships. Failing to communicate effectively (to listen and to compromise), too much self-righteous behavior or bullying attempts at control, making decisons on your own, failing to keep promises, and withdrawal from emotional and sexual intimacy are all that is needed to cause a marriage to disintegrate. Despite this being true, there are also 'large' events that can play a significant role in furthering the process of marriage breakdown. Abuse of any sort is one of these large events, and having affairs is another.
The common definition of what constitutes an affair seems to be sexual relations outside of marriage. This is perhaps a mistakenly narrow definition however, because it fails to recognize that marriages can be threatened by any relationship, sexual or otherwise, that threatens to break the bonds of intimacy and trust between spouses. A purely platonic friendship developed with someone outside the marriage can definitely cause problems inside the marriage if that friendship becomes more intense and intimate than the marriage relationship. In an important sense, any outside relationship that drains one spouse's ability to attend emotionally, sexually and/or intimately with/to his or her spouse is a potentially damaging affair. In this sense, online-chatting and cybersex can be real affairs capable of damaging marriages.
Affairs can happen at any time, although they are particularly likely to occur during the middle years of marriage. The popular conception is that affairs start after marriage problems already exist, and this does, of course, occur. Having an affair is, after all, a particularly dramatic means of separating from one's partner. However, affairs can also start in the context of healthy marriages as platonic extra-marital friendships that becomes passionate and sexual. It can be easy to mistake a new passionate infatuation for evidence that the older marriage relationship has died, but this is not necessarily the case. Rather, that an affair relationship feels more passionate than a marriage relationship generally only indicates that it is far newer than the marriage relationship. The passion of an affair will almost certainly cool if it is pursued to its logical extension. Someone who leaves their spouse for another man or woman may find themselves essentially back in the same situation they started in or worse when this cooling occurs. In most cases, if married partners understand themselves to be more or less compatible with their original spouse, they will not gain anything by pursuing another and may lose a lot. In any event, affairs don't always signal the end of a marriage, but they surely point to marital disconnects that must be addressed if the marriage is to survive an affair and become healthy again.