The reason is because as people we are teased with images, sexy shows, and of course, pornography on the Internet. Thus, many enjoy these pleasures while single, but, once married, suddenly all of those sexy images that turned us on... are now taken away. Once married the only sexy thoughts we should be having are about our spouse.
Many people go so far as to say that if you look at nude images or think about another person during sex when married then that is considered cheating.
Think about how unnatural all of those statements are, and in simple terms, over time, drive many people to "cheat" by watching porn behind their spouse's back.
When people do marry it is not supposed to be the end of looking at sexy pictures or thinking sexy thoughts or even controlling or suppressing the feeling of sleeping with another person. In fact, these feelings are completely natural. What is not natural is to suppress those feeling and shame a spouse for having them.
This does not mean that because we have those thoughts that we should act on them with another person, but simply understand them as a normal part of being human. Then, we can share those thoughts in the bedroom with our spouse (role-playing, fantasy, watching adult movies, erotic novels, sex toys...etc.) This will make the sex in the marriage all that much better and help to maintain a long-term monogamous relationship.
Marriage is not about suppression of sexual desire, but expression of sexual pleasure and freedom with the person who you love. A person with whom you can feel comfortable enough to explore your sexuality and not to be judged but embraced, that is what sex should be like in every marriage.