Is watching another man or another woman already a infidelity?

This weekend I had an interesting conversation with some of my friends about where infidelity starts. The opinions were wide-ranging. One of my friends said that it is already cheating on the partner, if he is just going to a strip club to look at other girls, some others (especially my male friends) said that cheating starts at having sex with another person and one of my friends also said that cheating for him starts at sex with another person, if there are feelings during the sex act. I need to say that I also agree that cheating starts with sex. If you are just looking at someone from the opposite gender, that is nothing special. I would rather say that it is IMPOSSIBLE not to look at people. Even I am looking at girls if I think that they are pretty or if they are wearing a very short skirt. It is not that I am lesbian; it is just the interest in people in general. So why should it be forbidden for men to look at other girls.
All I can say is ‘Girls don’t be so old fashioned with your thinking’ and ‘Boys don’t overstretch the bow’ (because the thing with ‘Cheating starts if there is more than just fun within the sex act’ is even for me a bit too much)