From a psychological perspective, it is in my opinion human nature for individual's to have needs of increasing self-esteem. Therefore, although I consider any sexual physical contact and directly seeking self-sexually gratifying behaviour (examples; directly messaging a person with a sexual focus/intention or seeking sexual attention through ones own behaviour) to be cheating, I do not agree that responding to flirtatious behaviour with a response that does not escalte to heightened sexual attraction as wrong. If another individual is to pay you sexual attention, therefore theoretically providing an increase to self-esteem (in my opinion a basic human need), I do not agree that this should be rejected or ignored. It could be considered a problem if one is not receiving similar satisfaction from ones relationship, but that is another story all-together. As is it fundamentally within the human nature of males to attempt to hunt a mate (for purposes of reproduction and our species continued existence) and for females to attempt to attract/lure a mate, I do not initially see a problem with this interaction.
In straight forward english, my example of what I find acceptable would be a woman looking at me in a club or offering myself a complement and myself accepting it or in reaction to her interest taking glance at the woman.
I however understand that these actions may be hard to judge and therefore people have differing opinions on this matter.