So, I have already talked about "growing apart" and "money" as two reasons people get divorced, the next one is more complicated, "relations" or more bluntly "sex". It is a very broad category, because it can be looked at from many different perspectives. First, are the parties sexually compatible. Were they ever, have they changed and are not anymore. No one knows what goes on behind closed doors, so, if they are not for any reason, that can be enough to make one or both parties look elsewhere or stop altogether having relations. There are also those instances where a party has shut down, that may be because of emotional reasons or simply a physical lack of desire. This too can obviously cause the other party to look elsewhere for intimacy and companionship. I have seen this with older couples especially. Then there are those instances where one or the other party looks elsewhere simply because they desire to do so or because of one of the above reasons.
Generally, it seems, that parties look elsewhere when the relationship has already had problems for a long time. Generally, its seems anecdotally, adultery is more a symptom of a poor relationship than a cause of one. Although, there are certainly instances where a party to a marriage takes free license to have affairs regardless of whether or not they are attracted to their spouse, have a good sex life with their spouse or have a good relationship with their spouse with no other problems. The statistics seem to show, since more women are in the workplace, that woman are just as likely as men to have adulterous relationships.
Can it be that this alone makes marriages more difficult, that the institution itself is more difficult to maintain with society's more liberal leanings, more openness and more acceptance of certain things. Only time will tell.