Nah man, stick to your guns. I think you really should tell him outright that there's no way it's happening and that he needs to look elsewhere if he wants to experiment like that. Three years is a long fucking time to pursue someone for one experimental night anyway, I mean you'd think he'd get the hint after being rejected the first few times.
Well, that's a bit unfair. There are people who've spent decades questioning their sexuality. There's just better ways to go about it.
It turns out it's much easier to make broad generalisations about people than to look at them individually. It's just something you'll have to get used to, sadly.