Guy who I am not attracted to wants a relationship. How do I let him down gently?

A guy who I would otherwise be interested in terms of values and goals is interested 7n a serious relationship.

THE PROBLEM:

I know that I won't be sexually attracted to him and it will be very hard not to desire other guys.

What do I say? I would like to be concise, firm and final.

Updates
1 y
So I told the guy I don't feel tue same way and he implied that I'm already sleeping with someone. I feel I dodged a bullet.
Guy who I am not attracted to wants a relationship. How do I let him down gently?
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