Compared to the doomed, ill-fated love of Buffy/Angel, Spike/Buffy, Tara/Willow and Giles/Jenny (apologies if your favoruite ship isn't mentioned here but these are the ones that illustrate my point best) a man and a woman with no super powers, magic or mystical whoosits between the does seem a little dull at first glance, but for that reason it's also the most realistic.
There are no romanticised notions of unobtainable love, no crazy metaphors of magic for intercourse and no fights for redemption, salvation or humanity. Just two normal people who fall for each other because they find themselves in close proximity on a daily basis. Like normal people starting relationships at work, college or a social group do in the real world. The most controversial part of their relationship is that it is inter-racial, which let's face it, is probably still more of a problem for some small-minded people than a girl doinking a vampire. Is it cool like many of the other relationships in the Whedonverse? Not really, but it is the kind of relationship most of us can reasonably expect to have. Minus the whole shoving someone through a portal to their death thing, or course 😀