Why did my boyfriend stop being intimate?

When my boyfriend and I started “dating” we would have sex all the time, that was pretty much the only thing we would do. Over the months, it steadied out and was around 2-3 times a week, which was fine with me. We would experiment with different positions and it would be completely spontaneous. Now, over the past 3 months, he has stopped being intimate with me at all. He hardly ever kisses me, hugges me, tells me that he loves me and the worst part of it is that he is acting like a woman in the sack. Always making excuses to not, or on the rare occasion he his in the mood, he vision of foreplay is “turn over baby, I want to stick it in”, then like two minutes later hes done and back to being a prude. I know that its not lis libido cause he is 23 and loves to look at porn and naked women. I havent changed at all. I havent put on weight or been over emotional, so why wont my boyfriend have sex or even kiss me anymore? the worst part of all is that last nite I hinted that I wanted some and he looked me dead in the eyes and said “naw, im good”. I thought he was joking, so I laughed it off and acted a little upset that he said that, then he kissed me for the first time in a week and when I looked at him again to see if we could start, you know… he just made up another excuse and went to bed. What did I do? the one part I left out is that he was a marijuana smoker for nearly ten years, but within the last few weeks, a little bit after this whole loss of intimacy thing happened, he stopped. could it be his reaction to not smoking anymore? help! how do I get my boyfriend interested in sex with me again?