Flow sex = sex by chance. Normally, she has sex with men she doesn't particularly notice as the opposite sex, and with the opposite sex in a neutral relationship, "in the flow of the moment," with the atmosphere, with the mood. Somehow, in the flow, "I was asked, well, whatever," she is swept away and ends up having sex in the flow. And then, unable to forget the pleasure of that partner's huge dick, she repeats flow sex in secret from her husband many times over.
