They are considered to be in a "sexless marriage".
According to statistics (1 in 5) marriages in the U.S. is classified as "sexless" based upon having sex less than 10 times per year.
The average married couple has sex 2-3 times per week.
Depending on the libido of each person in the marriage one may be content and the other silently suffering or engaged in cheating.
Communication is often mentioned as a solution but if one person has no interest in having more sex the other person can't force them.
A lot of people would rather cheat than run down to the courthouse to file for divorce, move out of their home, split assets, lower their living standards, become a weekend dad or single mom, pay child support and possibly alimony just because they're not having sex with their spouse! Cheaters also don't think they will be caught.