I've heard of many good people over the years who have falled into adultery. I don't think anyone plans on it. It could be a slow and gradual friendship of a man and woman who are always together and it creeps up on them with not planning it. I also think others do it without thinking at all, or just wanting to fullfill the desires of their flesh in the moment. Some people like the thrill and excitement of doing something in secret. It can feel like an escape from their hardships of life. One thing no one can really do is throw the first stone and look down on others who have. Humans are humans and are sinners.