「Flow Sex' '=Sex due to the flow of the situation'. Normally, with a 'man who wasn't particularly conscious of as the opposite sex,' an 'opposite sex in a neutral relationship that isn't liked or disliked,' and 'in the flow of the moment,' atmosphere, mood, it just happens. Vaguely, going with the flow, 'I was asked after all, well, whatever' and ending up having sex due to the flow. And unable to forget the pleasure of that partner's huge cock, repeatedly, in secret from her husband, she keeps having casual sex.
