For me, a lot of what makes this fantasy so hot is the thought that the mom is oblivious to her son's torment. In her mind, she's sneaking around perfectly; sending covert texts and emails to the bully, making secret dates, cleaning herself up before coming home. As far as she knows, she's doing a great job keeping everything hidden, all the while her son is miserable, knowing exactly what she's getting up to, but too scared to confront her about it. In these stories, the son's misery is mostly his own fault; it's his own cowardice and unwillingness to stand up for himself that causes the affair to go on longer. The mother's love for her son doesn't really go away in stories like this, but it does gradually fade, or become less important than her love of the bully's cock.

However, I know some people like the opposite, where the mother is aware all along that her lover bullies her son, and that her son knows she's fucking him, and either doesn't care, or purposefully joins in on the bullying. In these stories, the mother never really loved her son, but rather relishes in tormenting or mocking him. Personally these kinds of stories aren't for me as much, but I'm interested in hearing which you prefer.