Yes. I was married once. He moved out one day while I was at work and didn't tell me. I came home to a half-empty house and received papers in the mail that said, "You are being sued for divorce." He gave me no reason. I was told by Christians that at 25, I would have to remain alone for the rest of my life. I moved away from the area, and it later came out (a long time after he divorced me) that the reason he had left was to be with our 19-year-old co-worker. Once I was gone, they went public with their relationship. He had been dating her while he was still with me.
At last! The first question of your post, "What sexual sin does a married couple commit?" is getting right to the heart of the matter, thank you!
Some Christians seem to think that being married means they don't commit any sexual sins. And some may not -- I'm certainly not accusing anyone of something wrong if they aren't actually doing something wrong.
And yes. A Godly marriage is one man, one woman, and a sanctified marriage bed involving only the two of them in thought, word, and deed. But does that mean a married couple doesn't commit sexual sins? Maybe for some. But not for all.
* If a husband is fantasizing about his pretty young redheaded coworker while with his wife, is he sinning?
* If a spouse is watching things on their computer they have to hide from the other person, is that sin?
* If a wife wishes her husband would make more money, is attracted to their wealthier more powerful neighbor and starts to picture herself with him instead of her husband in personal ways, is that sin?
* If one spouse makes the other participate in sexual acts or uses equipment they are not comfortable with, is that sin?
* If a married couples like to use things seen as unconventional accessories in the bedroom, is that sin?
* If a spouse prefers to use other body parts or apparatuses other than the ones God specifically designed for sex and the other spouse is unwilling, but is pressured into it anyway, is that sin?
The answer to your question of whether or not people in marriage can commit sexual sins would have to conclude with your own personal answers to these questions.