The theory is that Spike and Buffy had sex at the party. A couple or so days later, the Siphoning caused Spike's semen to come alive inside of Buffy, and she conceived at that time. I don't know if Buffy douched, but that doesn't prevent pregnancy anyway (doesn't flush everything out, and some say it can even make conception easier.) Plan B, I dunno why Buffy didn't choose to use that, just in case.

Anyway it's a doable theory. Whether Joss is going there, I have no idea. If he does, he'll be my favorite fanfic writer of all time. ;-D