I've never quite understood it. "being sober" makes sense to me; sobriety means not intoxicated. straight edge means... well, different things to different people, but generally: no illicit drugs, no alcohol, no casual sex.
so I see in there a set of tenets that makes me think, self-respect, self-control, and recognition of dignity... but if you stop with the tenets of sXe you're just a scenester -- if you say you embrace a healthy life free of drugs and alcohol but habitually eat unhealthy food than you're a hypocrite. same if you pop a tylenol every time you've got a little headache or take nyquil at the first hint of a cold. I don't know why the casual sex thing is in sXe but if it's to avoid the risks of STDs but you continually put yourself at risk in other ways you're a hypocrite, or if it's to respect your dignity and the dignity of others and you treat people like shit in ways besides taking advantage of them sexually you're a hypocrite.
that's my 2 cents.