Open Marriage how do I Ask for it?

I want a open marriage, me and my husband have been together for 7 years married for 5. Im 29 and him 62. I work him retired recently. Our sex life has always been a problem, we have sex one time only never more than one time on a night. He always gets his and I always feel left out no orgasm unless he gives me oral sex which I enjoy but it not good enough. He says it's my fault because it's so good that he comes fast 5 minutes, always on my back or missionary style but he never tries to get it up to go again or attempt to please me and im tired of oral sex its become a cop out to good sex, and I feel deprived, I want to open up my marriage for good sex only. My husband says he knows that he is lacking but he has not tried to get medicine to fix it, also I don't want to hurt his feeling by telling him what he has already acknowledged, but I also don't want to cheat on my spouse so I need help please.