Good question.
On one condition. She gets to fuck me in the arse with a decent sized (but comparably small) strap-on. To sweeten the deal, we do this during a threesome with one of her hottest young friends who I was dead-keen on fucking.
To cut a long story short, it turns out I have an extremely sensitive anus/g-spot, and being fucked by my now long-time girlfriend makes me cum harder than I can possibly do so without something in my anus. We're talking projectile rope-throwing. I still enjoy it without, and do so 80% of the time we fuck. But trust me when I say that having a beautiful woman suck your cock, while an equally beautiful woman fucks your arse, is pretty fucken amazing.