There are few marriages that will survive a spouse that cheats. The partner that does this often justifies their actions. They will generally begin with blaming their spouse for their bad behavior. Rather than looking at what makes them cheat, they would prefer to come up with excuses. Modern society has made it easier to obtain a divorce. Rather than cheat on a spouse, an honest person would dissolve the marriage. This points to the fact that a spouse that cheats really is just looking for a way to avoid their responsibility.
Over time, it has become easier to find people willing to cheat. Some are married and cheating on their spouse. Others are unmarried, but they prefer to date and be physically intimate with a person that is married. They do this because they do not want a commitment to a relationship. They believe that being with a married person will take this obligation away from them. Whether or not a person is married, or knowingly dating a married person, they are hurting someone that has made a commitment. This is wrong and the people doing it are selfish.
The availability of fuck buddies on the internet has often been blamed for cheating in the last few years. This is an excuse, not a cause. People that want to cheat on their spouse have always been able to find a fuck buddy. This happened even before the internet came into being. No strings attached sex has been available for thousands of years. Some people are good at finding it, and other people have no clue how to find someone like that.
There are many excuses for cheating, but none of them really matter. Respecting a committed relationship is what a person should do. If someone wants to leave their spouse, they should go rather than cheat and make excuses. In the end, both partners will be happier if this is done.