Guys look at other women, it's an impulse... they really can't help it... and so long as they're not downright staring... or actually trying to touch the woman... there's no harm, no foul. You are with a man... not another woman... so naturally, his behavior will be different.
Porn is a recorded sexual event by strangers... they are more or less 'actors' just like any other movie. In real life the people in the porno probably don't like each other or even know each other very well.
A woman can watch a heartfelt movie about two lovers trying to overcome the odds so they can be together and really be drawn into it... The woman watching it isn't in love with the guy in the movie... and she doesn't know the woman in the movie either.. she is reminded of how love feels by the actions of the characters. She may may not desire the guy specifically.. she just likes how it feels to be reminded of love.
A man can watch a porno more tailored to his sexual instincts and really be drawn into it.... The man watching it isn't infatuated by the woman in the porno, he's not in love with her... and he doesn't know the man in the porno either... yet he can still be reminded of the intensity of sex itself. The characters in the porno are just there to create the semblance of sex... the guy doesn't desire the woman specifically... he just desires sex.
Asking a guy to give up porno's because of your insecurities is like him asking you to give up romance movies because of his insecurities. They're just people on TV... it's a fantasy world. You can no more lose him to the 'mostly plastic' wet dream on the TV.. any more than he can lose you to the handsome, sensitive hunk on the movie.
When you think about it... you have to admit... this is all kind of silly to get worked up over...
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen