I always see their relationship as being more about love than about sexuality. Regardless, I think the wilkipedia article is implying that sexuality was a common part of "Class S" relationships. I absolutely know I've heard of this before in that context.

I actually always thought that Wakaba/Utena was closer to what Class S is supposed to look like. Two female friends have somewhat of a romantic/sexualized dynamic without actually going there, then one of the girls finds a boy and "grows up" aka starts liking some dude and completely leaves her friend in the dust for the guy. Thankfully, Wakaba doesn't roll that way, she's still definitely friends with Utena after she has her Saionji drama. But she really isn't around as much after all that, is she? (Until she realizes Utena and Anthy are living with Akio... ) That could be just as much because Utena is getting more and more wrapped up in the craziness with the duels and all that comes with them, but the way I see it, the Utena/Wakaba dynamic is supposed to reflect a more stereotypical Class S relationship on some level to foil against Utena/Anthy which is pretty detached from Class S. The idea of Class S is that once they discover boys they'll pretty much forget about each other. With U&A, they end up having nothing to do with any portrayed male love interests, and instead their love for each other (though ambiguous) is deemed stronger and Anthy goes to look for her instead of taking the easy route and staying put. Really, they went through a lot to hopefully get their someday together, and in Class S the relationship is supposed to mean nothing in the long run and fall by the wayside.

I've spoken about this before, but I think that Class S is very similar to the old concept of romantic friendships in Western culture, if not the same.