1. Here's the deal tho. We're talking about a fictional show where ponies' physical attributes, that include mane and tail, are based on actual ponies and horses. There's no real need to get so overly technical about this. You're basically saying "because she has a long tail is more evidence she's heterosexual, else she'd have a short tail because that would make her faster" but thing is, if that really was such a thing, she'd have more trouble reaching Mach 10. Prolly never would've to begin with.
2. Here's the deal tho. RD's never been shown to care a great deal about attracting potential romantic interests. I doubt she keeps her tail long to look more beautiful for anyone.
3. The stereotype I refer to is "females keep long hair, males keep short hair." Again, you're overthinking things.
1: The fact is that there is drag from long tails. At the speed most horses run, it's negligable, but the drag gets worse as she goes faster. What reason is there for her to grow out that detriment?
2: So what? I'm currently not persuing any relationship possibilities; that doesn't mean my orientation has changed. And low tails are more common in mares than stallions, thus growing a tail out is an attempt to get male attention. She also has no problem with wearing a dress, looking pretty, or dancing with a guy. Those all generate male attention, which lesbians don't want.
3: In that case, it is a trend. I checked and found that mares and fillies tended to have longer tails than stallions and colts. There are exceptions, but why isn't Rainbow Dash one of them? As I said, it is a detriment to her carier goals. I looked at evidence in the show to come to my conclusions. Stallions have shown to generally be more interested in physical beauty than mares.
Circumstantial evidence is better than no evidence. There is nothing indicated that Rainbow is homosexual. I'm not doing this to ruin all the FlutterDash fanfics, just reccognizing that the assumption that she's gay is based on steriotypes. She's a tomboy that likes rainbows, so she must be gay, right?
I was looking at it from a psychological point of view. There are several behavior trends in homosexuals, and the subtle route was the best chance at getting a real answer.