Why would a man stop having sex?

My bf of 2yrs stopped wanting sex, weve been sexless for 3mths now, and i get frustrated about it. He used to want me 2x a day, went down to once a day, to once a week, now ZERO. Ive asked him about it, and he just said he got lazy. I even get rejected by him every single time I try to initiate it. Id get him up, but hed tell me to stop, and goes back to facebook. Ive talked to him about it a couple of times, and he just said, he got lazy, or got tired of it. I see that his facebook is full of half naked women, and just in general women dancing, or flaunting their body in tight dresses. And he is a top fan in a certain lady that takes half naked photos. I ask him if he doesnt see me attractive anymore, if he prefers masturbation, and he just ignores me. I was 95lbs when i started dating him, and i got up to 115lbs, i ask him if its because i gained some weight, still ignoring me. Sometimes i get so horny, i ask him for it, or try to initiate, and he just looks at me weird. I ask him if he would do anything about it, and he just said, "ill prob just sit there all dead while you ride me then." He said he isnt interested in sex anymore and he is proud of it. Should i leave him? I miss it so much when he still want me :'(