Despite that gems has no biological sex, they do possess gender relative to humans. All of them have female features, female voices and refer themselves as females... Steven is a hybrid so he is not an exception.
And since Steven Universe show is a metaphor of feminism basic ideals (equality and breaking gender roles). I think the biggest antagonist in the story will be a metaphor of "white male patriarchy".
Therefore I do belive that White Dimond is the only gem there with Male atributes and features and his personalitty will be the one of a male chauvinist dictator who manipulates the rest of the gems to serve him.
And the gems's search of colonize other planets and submit other cultures and beings is promoved by White Dimond as another metaphor of "white male patriarchy" in western socierty.
Even that the main character is a male, he has not the classic young man or boy behavior from past shows. In some way Steven has a personalitty more like female's caracters from earlier kid's TV shows as his empatty, kindness, and politness with other people.
Wich is a great step forward to break the gender beahavior roles paradigm in western culture, and show that a boy can be non violent and still a hero. And a girl can be a strong and brave leader. Also it shows that all of us has strengths and weaknesses and the way to achive succes is by eachother's coperation. Among othe great things like defense to homosexual couples and showing that love has no labels.
Anyway my theory is that White Diamond is a "male" and steven and his friends will gather all the gems and turn them against White Diamond as an apologia of feminism fighting against patriarchy.
Although the way I see it, White Diamond should be considered a foil to Lars and Steven.
I mean, a single male leading a group of females isn't a bad thing on it's own, but it is bad to justify it in a sexist and chauvinistic way like saying something like "Women are dumb. They need a male brain to lead them" so something else sexist like that.
Lars is the leader of an otherwise all "female" team. But he teats his team mates with respect, and understands that him being the leader has nothing to do with having a Y chromosome.
It's also been implied that Steven my one day become leader, but if he does, it won't be because of his sex or gender (and before you say it. Lion is an animal so he doesn't count as a "man" and Greg isn't realy a Crystal Gem).
Well I mean it would be a kind of contrived way of having a feminist message by having a blatantly misogynistic villain (especially a white male patriarch), especially seeing as the themes in question have been fairly subtle so far (I mean Jasper has already showed some classically patriarchal traits like being the abuser in an abusive relationship, resorting to violence more often than not, and an obsession with fusion which may or may not be a metaphor for sex while all without having to be male) and adding something so obvious as the main villain being a cliche sexist jerk would be inconsist with how the themes would be delievers. I feel it would be better to have White Diamond to be more of a traditional monarch who strongly believes that any form of change to social order could cause societal collapse (Just like in the Middle Ages in which the caste system was believed to be so important to social order people would inbreed rather than sully their bloodline by letting someone of a lower social standing in their family) .
Still I guess White Diamond could be male, I mean any Gem could take on male traits if they wanted to and as a Diamond nobody would say no to them so it is possible.
Overall: a sexist villain is fine but they should be subtle about it perhaps being a metaphor for a classic patriarchal role (such as a overzealous religious leader or a traditionalist monarch) seeing how some behaviours of those roles are still seen in people today.