How do I decline without destroying our friendship?

If you're not gay, you're not gay and your friend needs to accept that. You cannot choose to be gay anymore than he can choose to be straight. In my opinion, you need to be open and honest with him. Reinforce that he's your best friend and that you'll be there whenever he needs you, but that the relationship can and will be platonic. I can say that I know how he feels; I had a crush on my [straight] best friend for years and even made some passes at him. I'm 35 now and can look back and see just how stupid I was for doing that. But we live and learn. Support your friend, have a "bromance" with him, whatever...just reinforce the platonic nature of the relationship. He needs to respect that you're straight just as much as you respect that he's gay.