Sailor Uranus and Neptune are in fact lovers in the original Japanese version of Sailor Moon and in the Sailor Moon manga, so yes, they are homosexual or gay. This is how their relationship is in the Japanese version of Sailor Moon and in the manga. However, their relationship was changed to that of cousins for the English version of Sailor Moon for U.S./Canadian culture.
Naoko Takeuchi's interview at the 1998 San Diego Comic Convention:
The question "Are Uranus and Neptune really lovers, and if so why did you make them gay?" came from a young female fan who seemed genuinely upset and wanted a clear answer. The translator for the event left Naoko-san in a lurch, not fully comprehending the homophobia in the West that makes such a pairing unacceptable to some (for those in the don't know, Uranus and Neptune are two female characters who enjoy a close relationship). Naoko-san's answer was that, yes, they were a couple... but her answer indicated that she didn't at all understand the question from a Western perspective.
She obviously saw no "problem" in such a relationship, commenting only that Neptune was "feminine" and Uranus "masculine" and joked they had a relationship because "they had lots of time on their hands." In yet another question of gender, Naoko-san also came clean by saying that Haruka (Uranus) always has been and always will be a girl, dispelling long held rumors of hermaphroditism.
In an interview from the Italian Kappa Magazine #51, September 1996, Takeuchi said, "The relationship between Haruka and Michiru is quite special. I think the most important feeling in the world is friendship. The friendship between them is so strong that it becomes love. There's not only heterosexual love, but there also can be a homosexual love, in this case between two girls."
Further Evidence that Haruka and Michiru are homosexuals:
Episode 92: Usagi and Minako "stalk" Haruka, who they think is a cute boy.
Michiru: "You became friends with them?"
Haruka: "You're jealous?"
Haruka: "They're like kids, and they're very cute."
Episode 95: Haruka, when asked what her girlfriend's name is in a romance contest, says "Michiru". They later leave the contest because if they didn't, they'd win.
Episode 107: A boy, Masanori, has a crush on Michiru and gives her roses.
From Hitoshi Doi's synopsis:
Michiru: "Are you concerned?"
Haruka: "A little. Is it from a boy friend?"
Michiru: "Of course."
Haruka: "I didn't know that there was someone who was brave enough to fall in love with you."
Michiru: "It's not often that you would get jealous."
Haruka: "I'm not jealous. I just can't allow you to look at anyone else besides me."
Michiru: "Haruka, that's called jealousy."
SuperS special: Haruka flirts with a maid (who believes she's a guy). Michiru comes in and Haruka immediately ends it.
SuperS movie: Michiru comments to Haruka, when the fairies want to keep everyone as children, that "it's more fun as an adult, isn't it?", whereupon Haruka is flustered and doesn't reply. This implies that Haruka and Michiru are doing something together as adults which children don't do.
Michiru: "I think you eat too many sweets these days."
Haruka: "I never listen to that kind of talk outside of a bed."
Episode 180: Michiru flirts with Seiya, asking him to change her clothes. When Haruka tells him to get out, Michiru asks if she's jealous. Michiru offers to have Haruka change her clothes.
Episode 181: Haruka doesn't want Usagi to go on a date with Seiya but Michiru stops Haruka.
Haruka (to Michiru): You're hurting me!
Michiru (softly): Am I?
Haruka: I want you to touch me gently.
Michiru: Later, when we're alone.