Buffy is just over here trying desperately not to laugh at the most inept fighting she's ever seen. It's not working very well - in fact she's almost in full-on giggle mode, before her brain starts to click and put two and two together. Spike and Angel were drunk back in the evil days, Darla was mad because she walked in and 'found' them, and now Spike clearly doesn't want her to know what happened, to the extent that he'd punch Angel over it - now what could possibly be embarrassing enough to provoke such a reaction?

"Oh my GOD, did you two have sex?!"