Not to say that Saki is not gay (because I am fairly certain she is), but the author may preserve the subtext simply by implying that the marriage gesture is indicative of Saki's fear over losing a very close female bond. Keep in mind her piano teacher was obviously someone she cared for very much, but then she left after telling Saki she was getting married. Saki may fear that eventually Kanon will leave her for a relationship with a man somewhere down the line, and that fear is why she wishes they could be more closely bonded. In other words, the mangaka could simply write it off as Saki having abandonment issues, rather than her being attracted to Kanon romantically.
That being said, Saki is one thirsty girl. She gets worked up merely from being too close to Kanon. They should probably change the tag to at least "yuri crush" at this point, since it's not 100% clear if Kanon views Saki in the same way. Although, if the author does pursue a full-on yuri plot, I fear that it could end badly. With Kanon potentially rejecting Saki as a girlfriend, and by so doing, losing her most important friendship...not that Saki would abandon her, but the tension and awkwardness might just taint their friendship. And honestly, as much as I would love to see these two get together, I don't want to see the deterioration of such a lovely bond.