Quick note, as I am a dubtastic little spore, this is all manga based. Also, names used will be the names from the manga translation. There will also be spoilers.
Quick Background: Hunky boy band the Three Lights transfers into the sailor scouts' school. Seiya, Taiki, and Yaten manage to piss off Haruka, Ami, and Michiru for various reasons and Haruka doesn't trust them from the get go(she senses alien energy) Still, Mina and Lita are naturally thrilled to have pop idols at their school. Taiki and Yaten aren't particularly fond of the solar scouts, but Seiya frequently talks with Bunny and even sends her little cards about Galaxia. Eventually, the Starlights stumble across their missing princess and Bunny's friends are picked off one by one. Eternal Sailor Moon, Chibi-Chibi, the Starlights, and Kakyuu head off to the center of the galaxy to fight Galaxia. The Starlights are picked off right after escaping Lethe's river.
Star-crossed lovers? Admittedly, both Bunny and Seiya seem drawn/attracted to each other, but--
So, each one reminds the other one of the most important person in their lives. As for Bunny's behavior at the concert(getting all googly-eyed, etc.), Seiya is a handsome rock star singing a song that was basically written to attract Princess Kakyuu(a young woman who we can probably assume is about the same age as Bunny and her friends.) Said handsome rock star also has the same coloring as Darien...
The fact that Bunny trusts Seiya doesn't really mean much either. Bunny trusts just about everyone--she even saves Galaxia from falling into the Galaxy Cauldron because she "doesn't want to lose any more friends."
Also, after their deaths, Bunny never mentions the Starlights again. When diving into the Cauldron, she thinks of the solar scouts, Darien, and Kakyu(who she promised would be reborn)but not the Starlights. In the Galaxy Cauldron, she never asks Cosmos Guardian about the fate of the other scouts.
In conclusion: Bunny and Seiya are friends. And, in the manga at least, they've both got bigger things to worry about than getting romantically involved.
My opinion: At the risk of getting flamed: thhpppth! I don't get the appeal of this at all...you send a girl a bunch a weird cards and suddenly you're meant to be together...I also doubt this one would have half as many rabid fans if Seiya was all girl, all the time in the anime as well as the manga.