He doesn't initiate sex...

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For starters, YES I already talked to him about this and I got nowhere. He rarely initiates sex anymore. We live together. I initiate it and he still goes along with it... I just don't understand why he never tries to have sex. I asked him and I was like "well, are you bored? Uninterested? What is the reason???" and he claims there is no reason and he doesn't know why. He said he is not bored of me and loves me. He just doesn't try. Advice?