The irony of all this, at least partly, is what the original character concept for this character was. Lauren Faust wanted her favorite pony Firefly to return to the show, who had a pink coat with blue mane and tail, but for copyright reason she could not do so. As such Lauren decided to put her view on Firefly's personality into the G3 pony Rainbow Dash, a very girly pony it should be noted that liked dresses, meaning people using Friendship is Magic Rainbow Dash's appearance to justify her being lesbian is amusing.
Another thing that should be noted is that originally Rainbow Dash was never supposed to have rainbow colored mane and tail. Rainbow Dash was originally Firefly, with a blue mane and tail... but because of copyright issues Lauren Faust had to change her initial designs.