I think you should tell him how you feel about him and how is actions are affecting you. Please explain that his talk of possible marriage are not humorous because you do have feelings for him. Even in long-standing friendships, it is important to have boundaries. You need to set one and you need to open up about your feelings. Hopefully, he is a good enough friend to sense the deepness of the problem and to have empathy for you and will also seek to help you find someone with whom you can fall in love.

I was in a similar situation years ago and did not know how to handle it. I had empathy but did not want to lead her on. I continued to provide emotional support until she broke off the friendship. She is seeing a guy I don't really like much and who may be homophobie. But, at least she is no longer alone, and for that I am grateful.