Umm... I'm not sure metaphor is the right term here. I mean, Buffy and Angel aren't metaphors for the exes that never quite ex, they are those exes. Kafka's bug guy was a metaphor. Bangel's drinking scene was a metaphor. The whole Angel-turning-evil thing was a metaphor. Heck, vampiric blood drinking is a metaphor -- especially when used post Bram Stoker, as his influence on the genre is pretty much set in stone and impossible to escape (not that I'd want to). To be a metaphor, something normally used to designate one thing must be used to designate another. There are many layers of metaphor in the Buffyverse, but I'm not sure this is one.