Some background: My wife and I have an open relationship. At first this was because she is bisexual and wanted to be free to see women for sex, I felt that this was one sided and we agreed that as long as we are honest with each other then we can do what we want (within rules). The main rule is that neither of us would see the same person for sex more than twice (this didn't apply to her and girls, just guys) this rule was to reduce any attachment forming. We are both comfortable with the idea of sex for fun so our relationship has stayed very solid over the years, either despite our arrangement of because of it.
A few years ago I began to get propositioned by women that we had known for several years and had reached their mid to late 30's (As I had), or more to the point they all had kids who had reached teenage years. It seems (from my perspective) that after spending their peak years looking after young kids, once the kids got to be teenagers the women found themselves with time on their hands,the ungrateful teens making them feel like they have wasted their youth and often in the passing years their husbands had become distant physically.