He met Buffy at a bar then met her mom. He ends up married to her mom. They end up having an affair.

Hi, it makes me think of «Love takes time» by Brat https://spikeluver.com/SpuffyRealm/viewstory.php?sid=23720
I haven't read it but here's the summary: Buffy has returned to Sunnydale to bury her mother after having been estranged from her for the past five years. The cause of the problem was her mother's marriage to a certain younger man named Spike. Having to face one another for the first time in five years, Spike and Buffy find that they might need each other more than they realize.

Probably not this one (and not Brat's either after looking at it) but the theme is pretty much the same. «Older» by NautiBitz, summary: A novel-length super-AU (everyone's human) story in which Spike is Buffy's new stepfather and they can't help but have a torrid, sekrit affair and fall crazy in love.
Only available on the author's website http://www.nautibitz.com/fic/older/index.html can be downloaded in PDF.
It would help to know if the fic you're looking for is short, long, very old or new-ish.

OK, after reading this topic I'm eventually pretty sure the story you're looking for is Older by NautiBitz.