What are we all meaning here when we say "normal?" A human? A civilian who isn't aware of the supernatural? Something else? I don't see how any of us can say definitively that Buffy can't make a type of relationship work if we haven't even defined what they type of relationship is. Even then, I don't think we can make that call. We may, as individual fans, believe it, but it won't make it a fact or canon. What makes a relationship successful anyway? Marriage? But that has a 50% chance of ending in divorce.
All we know is that Buffy has a number of long term relationships that ended roughly. That's it. Does the fact that all those relationships have been with men who are part of "her world" lend to a theory that she can't make a relationship work with "unusual" men?
Whatever theory you draw from the facts are just that, theories. Buffy's dating past doesn't really suggest anything about what type of relationships she can have. What they do suggest, is that slaying, being the slayer and making choices that compromise the life you have with someone, has effected her romantic relationships. But even still, it isn't definitive.
---------- Post added at 12:50 AM ---------- Previous post was at 12:40 AM ----------
[MENTION=12614]thetopher[/MENTION]
OK, we take Xander but even then he has lived and extraordinary life and has had the most unusual experiences to other men his age who know nothing about what goes bump in the night.
If I was to meet Xander and start going out with him, and he told me/show me his life, I'd think he was the least normal person I had ever met.
I suppose, that is another variable to add in: are we talking normal from our perspectives or from Buffy's?