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Take up watching porn yourself ... gay male porn. Watch it all the time. Leave magazines out. Leave tons of it in your browser cache. Leave video tapes or DVDs running. Ask him if he can't just adjust and get into it. Tell him he's prude if he doesn't want to watch it with you.
Granted, this may not work on some men, but most straight men are as horrifed by gay male porn as women are by male/female porn.
There's a reason for this: porn feels very different to many men when they see another male being "used" for pleasure in the way they like to see women used for pleasure.
I swear it works.