When he looks at this girl, you fear
a.) He'll find out about your thoughts or
b.) he'll do it too which you'll feel hurt.
O ne rule of thumb to help you avoid this: If you can't take it happening to you, don't do it to them. sure its your own private thoughts but our thoughts always end up reflecting off those you care about. The other thing to consider is if he loves you this means he can read even your slightest movements in relation to your thoughts and words. He can tell when your lying and when your thoughts stray from ONLY him as a lover (on the mental front). He picks it up intuitively, a simple conversation on the phone will tell him a lot.
Fantasizing IS The On-Ramp To Potentially Cheating
If you indulge in fantasies of other people intimately, you will change (body language, thoughts, filters). Realize we're creatures of manifestation, our thoughts become real, they shape everything in our lives, if you think it you will see it eventually manifest in some form in the real world. It doesn't have to be obvious or manifest in the way you think- it actually ends up manifesting in negative ways such as losing patience with your partner, losing interest, and more.
You will subtly act different and it will snowball to more and more feelings of guilt, ramping up anxiety (that your causing to yourself!) which you will see projected on him/her when he/she does anything remotely triggering to your memory of what you indulged in. From looking at a girl for one second too long to saying thank you to a hot waitress with an innocent smile on his face and a twinkle in his eye it usually manifests in guilt and jealousy.
He will do nothing wrong but you will over analyze his actions because all you'll see is your own guilt. If you start accusing him of cheating, looking at other people, and the like it will not end well- many men have cheated for no other reason that their lover pushed them to it with irrational jealousy. They figure if they are going to get judged and hung they may as well have done the crime (that's no excuse but it is reality).
The whole thing in this scenario is your own fault for doing what you'd not want him to do, even if it was innocent it strikes fear into you when you see him looking, and you label his actions according to your fear/guilt. Solution? Don't do anything you don't want him to do and liberate yourself (bared repeating).
Thoughts are not benign, and this scenario experienced personally should prove it! It's not that he will know you thought intimately of someone else, its that YOU will know and build a filter, and strain the rest of the world through it!
This KILLS passion, which is a free agent. A liberated feeling of trust breeds trust.. so monitor and be responsible in your thoughts as well as your actions, thinking is no different from doing it.
Rule of thumb, if you can't tell him about it, then its a toxic thought, so stop before it hurts your relationship! You should also realize if your fantasizing about other people, that alone is a sign that your relationship is likely stagnating in some way.
Passion in a lot of relationships are infected by this seemingly benign type of thing. Think as if everyone can hear you because eventually through your actions, fears, filters, and expectations... they will eventually manifest so they can hear you anyway... And the consequences can be very humbling.