This is an interview with Naoko Takeuchi that was published in an Italian magazine (Kappa Magazine 51, September 1996). 
As you will see for yourself, in this interview Naoko clearly states that Haruka and Michiru are in love.
It was originally up on my other webpage, the Palace of Moonlight, which is now closed, and which was the original source of this interview.
It now can be found on many other webpages, including Save Our Sailors (which has notably mistranslated it to make it unclear and which, in addition, has given no credit to me).

Q: You've been into shojo manga for a long time and you can be considered as the most famous author in Japan. What's the difference between the old love stories and the ones you write now, full of action and fighting? 

A: Love is essential in every manga I write. Even if in Sailor Moon I inserted a lot of fights, nothing has changed since my previous works. 

Q: Which Sailor senshi do you identify yourself the most with? 

A: There's a part of me in each of the ten senshi. According to the circumstances, each senshi speaks with my voice. No, there isn't a character I feel more like me. 

Q: The relationship between Haruka and Michiru is quite ambiguous. Can you tell us about it? 

A: 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. 

Q: Haven't you had trouble for this choice because of your young public?

A: In Japan, strong girls are very popular. The tradition of my country has in the Tarazuka, the Japanese theater in which only women take part, the maximum level of feminine emancipation. These actress cover all roles of the plays, even the male ones. 
I was inspired by them to create Haruka. It wasn't easy to make children understand how there could be true love between two women. 
Haruka is a tomboy, she talks and dresses like a boy, and therefore it's natural she falls in love with Michiru. 

Q: Differently than other shojo, Sailor Moon is followed by a lot of boys. An exception to the rule? 

A: Yes. The senshi are very sexy, and boys like it. In Japan, moreover, boys are quite weak and they search for a strong partner. They want to be dominated, and the senshi are ready to do it. 

Q: A winning choice. And a lot of authors have tried to follow your way, creating small and big clones of Sailor Moon.... 

A: Thank you. Maybe what a lot of authors don't understand is the masochistic component of boys. They write love stories that are now outdated. 

Q: Will there be a sixth series of Sailor Moon? 

A: I don't know, but for now I don't want to end it, since SailorStars has just begun being aired.