Ahem, here thee here thee! All persons believing that is a lesbian is hereby decreed WRONG by (her majesty) Lauren Faust herself:
"Rainbow Dash has rainbow-striped hair because of her name and because she is very interested in sports, specifically flying. She is a tomboy, but nowhere in the show is her sexual orientation ever referenced. As we all know, there are plenty of straight tomboys in the world, and assuming they are lesbians is extremely unfair to both straight and lesbian tomboys."
Here is my source: http://msmagazine.com/blog/blog/2010/12/24/my-little-non-homophobic-non-racist-non-smart-shaming-pony-a-rebuttal/
Note that she doesn't say Rainbow Dash's sexual orientation (and really, does it matter?), simply that no one should assume she is automatically a lesbian. Just because the female to male ratio in Equestria is 9:1, that doesn't mean every mare is a lesbian. So I advise this little tip to live by: Rainbow Dash is not a lesbian unless it aids the joke, story, or Shipping thread. Can we all just agree on this? Please?