i think the reason Andrews sexuality comes through as a joke, is because everything about andrew is kind of like made fun of in everuy way
we never get to take him seriously
and the fact that he doesnt sort of openly come out with it is supposed to be amusing because hes so incompetent he cant even get his sexuality right ( im saying right, as in knowing it, not right as in straight is good, gay is bad you know)
and xander being gay
well as amusing as it was to hear him say 'gay me up' i cant really see it
for willow it was a natural progression
wicca has always been about the strength of women and coming together in strength gaining power and she took that furtheer, seeing the strength of spirit in willow etc and realising she was gay, like learning about the magics and the power within herself allowed her to open up and see her true self and stuff
im probably saying this all weird
but basically i voted for the either way it doesnt matter option
because he could gay or straight and id still love my xander!