ok so here's a question for you. This actually happened to me. You can question HOW this happened but I can tell you know, I have no answer for that.
So back when my kids were ... maybe 5 and 3, my then-husband had an affair that lasted 2 years. I didn't know they were romantically involved. I knew Beth, I knew they were good friends and they did a lot together. I didn't know they were sleeping together. Toward the end of these two years, he asked me for a divorce - at the time he intended to divorce me and marry her. Shortly after that, he broke up with her for some reason, and we didnt' end up getting divorced until 2 years after that.
Anyway, during the time they were together, she considered him "her boyfriend." They spent a lot of time with our kids. I was showing (horses) competitively at that time and spent 30+ hours a week at the barn, and I was glad that then-DH and Beth watched my kids during this time. I spent very little time with my then-husband, that is true, we did nothing together at all. He had a social life, wtih a network of friends, that included his girlfriend but I was not part of this network. We had separate friends. They socialized as a couple with these friends, and brought my kids with them often.
Was the girfriend a stepmom to my kids? She fully expected to marry her boyfriend (my then-husband), once he disentangled himself from his doomed marriage.
(this is just a question for discussion - I really dont care one way the other, since this was 15 years ago).