I've got myself in a bit of a situation, help?

[GagasBadKid]

So I'm a lesbian and recently some guy I'm best friends with said he loved me and I said I loved him back, I thought nothing of it because he always says he loves me. He lives in Australia and I live in England. So today he asked me to marry him on facebook so I agreed and accepted his marriage request. We skyped earlier and he told me I was beautiful, sexy and perfect and I said "thank you, you're adorable!" He also said I'm the girl of his dreams and I flirted with him just to be friendly (I'm known to be a bit of a flirt) and now he's just asked me out... he knows I'm a lesbian so what should I do/say? I don't want to upset him and I'm scared that I may have led him on... I feel terrible :/ help?
Wait, I forgot to mention that he used to live in England, he moved away 6 years ago.