It appears the two girls were pretty well liked by the other students, and these other students wanted to make a statement that sexual preference should not be an issue of judgment. While selecting the two to rein as king and queen might seem an over-the-top way to express this feeling, that the students chose it speaks to their priorities: de-demonizing homosexuality is more important than adhering to the rules of gender roles. In reality, only males are kings, but it's not one of those rules that, in a fictional situation, is going to suffer by breaking it.
That any student, male or female, is crowned king carries no more power than crowning them cod fish. It's a fictitious title, and as such can be constituted of whatever one wishes. Think king should only apply to males? Fine, but why make their selection one of popularity (by popular vote) when real kings are seldom chosen this way? Isn't that breaking one of the "King rules"? If you're going to consistently break one of the rules of kingship, hereditary monarchy, then why can't a student body occasionally break the gender rule: males only?