I think it's pretty gross myself, but there is a simple explination for it.

In fan fiction and general fandom, lines of love will be drawn where there are no lines present in the game/show/etc. Particularily in yaoi. It's like someone sits there and goes "Hmm... now, who would make the most irreverant and impossible match?" and then writes a story about it.

I could never see Rikku and Auron getting together, and age has nothing to do it it. The personalities are just to conflicting. To be honest, they remind me a faint bit of Rinoa and Squall, who I personally think are an impossible couple. Oh well.