while the "disgusting" argument is quite strong, and yourself and this man are blood relatives - have you been brought up quite closely? Is there a strong family connection there, or is he more like a mate?
Personally i wouldn't bed my cousin, but i know its not unheard of. Reading your post, my main concern isn't actually that he's related to you, but that you're in a vulnerable position right now, and maybe jumping into bed with a man - any man - might not be such a good idea.
does he love you back? what will your mum and his mum think? you'd have to be 110% sure it's true love on both sides and not just rebound sex before you potentially go and cause a huge family rift.
Good Luck!