Is it something done in secrecy or do you let him know when you're going to watch it? Is it just videos/movies or websites where people can talk to others via web cam?
The two of you are most likely aware that men are visually aroused/stimulated. It is a fact about how the brain works. There is a study that outlines the difference b/w how the brains of gay oriented men differ from those who've identified their partner preference as heterosexual (here).
It may be that your libido is higher than his which can also contribute to your interest in the videos. Something to be mindful on your part would be if the behavior had become addictive or if it is within the manageable and healthy parameters.
Does your partner feel that the attention you allocate to this is taking away from the attention or affection you could display to him? Would he be all right to watch a video with you? Does he compare himself or the relationship dynamics (sexual intimacy) to what is portrayed in the videos? All of these questions would have to be clarified by him to determine what exactly is the most upsetting part to him in regard to your behavior.
He decides 1) how to interpret your behavior 2) how to feel and 3) what to think about it (see it as cheating, taking time away from him, a distraction to the relationship, and 4) how to view himself (esteem/insecurity issues) Was there infidelity in his own family that could be also triggering his reaction (even subconsciously)?
Perhaps he could write down a list of A) What happens and B) what emotions and thoughts it evokes within him. Then he can look at associations such as: When you do this___________, it reminds me of__________, I feel_________because__________ and spend some time in self introspection and claim some ownership about his own reaction/interpetation of this situation.
Your partner's interpretation of your behavior can be related to his self esteem issues if what you're doing is simply watching and not trying to connect in anyway with other men. His insecurity can manifest in him feeling "betrayed." No matter what you do or say, he is the one that ultimately would have to address his self esteem and how it may be harming the relationship if left unresolved on his part.
This book is more about giving you some idea as to why he may feel this way depending on whether or not you also interact with anyone online or develop feelings for -
Love Online: Emotions on the Internet