How can I reassure my autistic best friend he need not marry me to keep me?

I have a guy friend with moderate Asperger’s, who says he’s not attracted to me. He claims his love is platonic; and he admitted that he had planned to marry me, because he has a fear of abandonment and marriage is the most logical way to lock down permanence with a person. I had feelings for him and I have milder Asperger’s and milder abandonment fears but I don’t entirely understand his train of thought. It’s come up time and again in our friendship, where he’s tried to force attraction, which I never asked him to do.
How can I reassure my autistic best friend he need not marry me to keep me?
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