Why does my husband watch pornography?

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I am in a three-year relationship and I live with my boyfriend. We love each other but I have noticed that whenever we get intimate, he watches porn on his phone so that he can reach his climax. He doesn't know that I am aware of this because he actually puts it under the pillow and uses earphones in the process. I am somehow worried because I don't understand why he has to watch such things to get satisfaction. Is it that I am not good enough for him or does he not find me attractive anymore? I really don't know what is going on here and I don't know how to confront him about this. Please advise me. {Cynthia}