It doesn't matter why, even if the "why" has been specifically conditioned into the both of you just so that the person selling the diamond makes a profit? That seems a bit... I don't know... tragic.

(To be clear, though, I'm a nerdy gay like the parent commenter, and I can't fathom a situation where either my boyfriend or I would ever want the other one to buy a diamond for any reason, so there's probably some component of the dynamics here that I don't fully grasp.)