It's like eating the same food 3 times a day for the rest of your life......it's unrealistic and he's human and your human mind works way to fast to control every thought of interest when being in the visual range or mental awareness state of another, other than who you're with. Some think that unless you shut out all thoughts and never think about or have fleeting thoughts about another while with someone, you're being faithful. Let's face it.....if that were actually true, that person would be "dead"
That doesn't mean you can't develop feelings or attachments/attractions to others on line and actually begin to pull away from the one you're with....but perhaps there is a deeper reason that the person is doing that other than boredom. Maybe the relationship is cracked, or the person you're with wants to move on without you.....
Is it cheating?....Most define that as an overt act of actually having physical contact...but the advent of the computer and these sites has changed that dynamic a whole bunch, because 1 out of 5 relationships now begins on the internet, and clearly you can fall in "love/lust" quite easily on these sites looking at the right photos or reading the right words that gives you that "heart pounding" attraction we all like so much. I would say you can be crushed if you read something on the computer where it clearly shows the person you're with has moved into some "emotional" relationship and to be that is cheating....but fantasizing over photos and voices is so common and expected at some point, because as long as we can dream or imagine or fantasize....it'll happen.
I've always lived by the rule that there is two types of sex...physical and emotional. I don't worry about the physical type if she's open and honest and just out for a bump and an ego boost and great sex and tells me about it (I like being a cuckold)....but the emotional one, even with my knowledge is a crusher, because that means she has opened her heart, and not just her legs. Some are capable of one with out the other, and some are not......
When all else fails maybe talk about it...and ask what he's getting out of these contacts, and what he needs from them that he's not getting from you....and if it's more physical that emotional, maybe try to get something out of it too.......or you're libel to be alone in the future.......or YOU find another who maybe is looking for or feels the same way you do.......and hopefully your life will be better in the long term.