Why is he not interested in sex?

[missA]

I've been with my fiance a little over 2 years. We live together and our relationship is great. Except our sex life. He's never really been all that interested in sex. We've talked about it and he just says sex isn't important to him. I've tried initiating it many times and sometimes he'll do it. But most of the time he says he's tired or has a headache. Even on weekends he has off. We are both 24 and having sex once a week sometimes as once every two weeks just doesn't seem normal. It has become quite frustrating to me and honestly has me wondering if it's me or my weight. I know for a fact he isn't cheating or getting his satisfaction to porn. We both have same views on porn and porn is blocked on tv and internet. Plus I have access to his computer at any time to make sure it isn't going on. Now this isn't something that has just started. Ever since we first started dating he never has been overly interested in sex. We both do believe he has some issue with ed. He is hard to arouse and he climaxes very quickly. An average sex session lasts maybe 3 minutes at the most. And I do my best to assure him that it's all ok and that I love him and have no problem with it. I know this could be some issue with him not being interested. What are other possibilities? What can I do to help? I'm so frustrated. Sorry this was so long.