I'm not trying to sound harsh, ok......I don't know if you noticed, but quite often there are times when someone posts something you then post something vaguely similar or the exact opposite. This kinda seems a bit spamish, just to let you know. I know that you are honestly trying to look for fics, but wow does it come across like you stalk the find the fic section.

But on that note:
I haven't read anything like this. Maybe borderline similar. Only because Spike just being effected sounds a bit strange if there isn't a story told a head of this spell being done. I know in Jackospikes story this is the case
Chosen After Death by JackOfSpikes Rated: NC-17But no marriage per say just a joining as mates.
Slaymesoftly has Just a little spell rated NC-17What if Spike and Buffy got REALLY carried away while under Willow's spell? A long fic with one of my shmoopier endings. Original characters, Xander-bashing (funny Xander-bashing) and surprises from the Council for a claimed Slayer and her vampire.or try
Something Blue, Something More by Opal Rated NC-17Immediately after Willow cast her "My Will Be Done" spell, Spike and Buffy deal with the aftermath of sleeping with each other during the spell. The Slayer is determined to have a normal life, with a normal boyfriend. Spike leaves town, but she finds out he left her with something more. BSV Challenge # 93What you're looking for sounds more like Opal's story.
Only Spike effected would have to have Buffy interested already or written otherwise if it's canon Buffy she would be absolutely horrible to him...well that's how I see it.
I've seen more of Buffy being effected than only Spike effected. Like I said Spike effected only HAS to have a story first AND Buffy must love or want him first. I find that canon Buffy would have either chained him up, told the others he was bespelled, or dusted him. So not too sure about this one, just to enlighten you.
I know interesting concept but you might be mixing a lot of stories together or possibly giving an idea to someone to write about! Good luck!