what elements of female character appearance would get you to whisper them

G.I.R.L.
Guy In Real Life

As some have said earlier... I think it's mainly the pink.

Most stereotypical MMO guys (the type that would hit on a female avatar at least) couldn't imagine a dude using pink on anything.
So they just assume that it must be a sign that you are a real girl and not just a dude playing a girl.

i have no idea what prompts people to whisper my characters, neither of them are dressed skimpy, one has the dramatic extrovert chest, and the other has the sensuous dress, but both have long pants, full boots and gloves, hats, everything else is pretty much covered....one is chiss, the other is human with the long white hair....

Aww now I'm all upset that none of my female characters have been harassed. They all wear non-revealing outfits too.

Maybe its down to the pink dye, I've never used it.

"What is it?"

It's perverts being perverts. You had to ask?

I believe it's the lekku. You know, soft, sensitive, prominent body part which comes in pair.

You may be close to the mark there. I've several female characters, 1 human, 1 chiss, 1 pureblood and 1 mirialan.

The pureblood looks like a body builder (with the most minimalistic corresponding outfit I can find), and NEVER has had any comment about 'her' looks. Neither has the human or mirialan (body type 2 and 4 respectively).

The chis however (also body type 4) I recently outfitted with the polar outfit, dyed it pale blue with purple on the rims, and the first time I took her out to Hoth I immediately got one comment, which mostly was about her looking so in-place and not about her 'sexiness'. Funnily enough my Ashara companion with her red skin and white bikini and white knee boots I also got a remark about how sexy she looked...go figure...

And why I even have female characters? Simple, not so I can get her to wear skimpy outfits, I have no real sense of dressing up anyways, but just because I rather look at a woman's behind than that of a guy...even though I do have male characters and my main actually being a male.

It's not a bad theory but one might need a lot of play time to prove it. I've not gotten a single flirt or comment on any of my SWToR characters, male or female. The only MMO character of mine that people have hit on is my female blood elf paladin in WoW. She does fit your theory though - always looking good in heavy full plate (back in TBC she even refused to use some gear because it made her look silly). No chainmail bikini there, no Sir!

My female Jugg has actually gotten a fair few OOC kudos for the fact that I intentionally made her a cyborgy, corrupted, cracked-skin and ugly mess.

Even got a couple of requests for some pretty racey stuff, but I don't even ERP, let alone do stuff involving hooks and whips and chains.

PS. How do you even do that on an MMO? Actually...don't answer that.

Reading this thread kinda makes me want to create a new character dress her in pink and just stand in the fleet and watch the whispers coming in. On the other hand I _do_ have a female twi'lek that absolutely does not get (as in understand) any innuendo at all, which is very much fun as well. (Makes me wish they'll fix the chatbug, but that's another matter)

As for twi'leks being teenaged boys' favourite I think that's also a bit related to them supposedly being "more nimble" or anything. Though that's in-universe, so I might be wrong about that.

I'd like to also add that as a member of the male populace I personally don't get the preference of Bodytype 4. In my eyes it's just too bulky. BT3 isn't a favourite of mine either, but my BH got it, because it fits her character - and it leads to some interesting conversations as well. Makes her stand out quite literally.

To return to topic (sort of), as somebody else pointed out already, whispers are... semi out of game, so it's more of a mindset of "That could actually be a girl." and most of the time (but not always) out of character anyways. It's also why I seldom whisper people and if it is in character simulate a holocall or something along the lines assuming we're not close anyway (i.e. in the same cantina or anything).
Long story short: I would probably ignore/not react to those "random" whispers unless they're really creative. Or I'm really bored.