it's true that sex addiction is diagnosable as a mental illness. It's not totally clear to me that the OP is describing someone who is actually a sex addict (rather than just someone who is going out and having affairs). But it does seem that the OP was gesturing toward ALL moms who have affairs as being somehow mentally ill (most of the OP's post was referring to a GROUP of moms who are all having affairs for whatever reason).
It's interesting -- I've seen several threads on the DIS like this and it does seem that there is some notion among (at least a proportion of ) disers that one (especially if one is a woman

) has to be mentally ill to sleep around and/or that it's such unbelievably strange behavior that a person who does it must be totally outside of the range of normal.
Interestingly, when you look at some numbers, it's pretty common. Depending on the source, we see about 30-40 percent of people have extramarital affairs at some point in their lives. I would think, then, that we would have to agree that extramarital sex is a variation of normal sexual behavior (whether or not we think it's a good or bad thing is a totally different story).
Anyway, I don't mean to pick on the OP. I just think it's interesting that it is treated as a shockingly abnormal thing, when it is actually very common.
As a side note, I would think that the reason that "moms" have affairs is similar to the reason that anyone else has affairs -- they're dissatisfied with their partner, they have a robust sex drive, they're bored, they're annoyed or angry with how their partner treats them, they're satisfying a momentary urge, etc. I wouldn't think that "moms" need to have "gone Crazy" to have an affairs.