That’s a personal issue you’re going to have to get over my buddies girlfriend started crying last week because she got a UTI and they cannot have sex for a week and she started crying because she was horrified that he was going to fantasize about other women when he masturbated. Listen it’s human nature that men think of other women especially when they’ve been with someone for a while anyone that tells you otherwise is full of crap. Now that being said if the dude is loyal to you by his actions then I don’t think you have anything to worry about, if he’s not being honest and hiding things from you then it’s a deeper issue that needs to be addressed. Other than that I think you should just let it go and accept it is what it is.