Most Helpful Guy
When I have a girlfriend, I will NOT cheat on her. I won't be sexual to other girls at all, and sometimes fend off slightly sexual advances.
BUT... that doesn't mean I won't find other girls sexually attractive. I do, and that's largely due to biological/genetic "programming." I won't act on it, but I'll definitely look and appreciate a nice body or pretty face. And I will still watch p*rn, but keep in mind that guys watching p*rn tend not to see the whole girl as much as individual body parts (porn is NOT real life). P*rn is fantasy.
Many girls I've been with totally get that (a few have been a little insecure), and some would even go out of their way to point out hot women for me to look at. They know that I may look, but I'll only be touching them, and they'll get the benefit of any extra testosterone that got stirred up by me looking at the other girls.