To keep the job the Slayer must be married at some point...

Instead of a Cruciamentum (or in addition), there's a requirement that every Slayer must be married, and it must be to a vampire. The Council would love to change this but can't... In fact, it could be the reason for the Cruciamentum, if you'd like, to get rid of Slayer before the requirement kicks in.

At some point in the Slayers life (pick a birthday, milestone, etc.) every Slayer must marry a vampire to stay the Slayer.

Of course Buffy first asks Angel or thinks of Angel, but that's ruled out. Either because he says no. Or because they'll have to consummate and that's a no go. Or he's unavailable and in Pylea when it happens and there's a time restriction. Or he's unable to for some other reason (Darla already claimed him, he's been married before while human, etc.)

For whatever reason Spike is the perfect fit and agrees to do it. Maybe he has to marry someone too in order to ... ? Or he just agrees because he loves Buffy. Or they threaten him. Or bribe him. Whatever...

Spuffy have to be married for a minimum of XX time? A year? Three years? Whatever. Ultimately, of course, HEA they fall in love and decide to stay married.

If you'd like the banner, just pm me, please.