Kex or kecks anyone?
I would have just said that that’s probably the origin of the name. And that most people can at least see how panties are a diminuitive form of pants. They’d just be tight ones (like long underwear), and would be made smaller by reducing the leg length until they almost weren’t there at all.
Also, male underwear is often (IME) referred to as “shorts.” Since there’s a type of outerwear with the same designation, maybe that would make the diminuitive more clear.
Still, my own reasoning would be that most people don’t give a whit where the words for things come from. If they learned to call them panties, they learned to call them panties.
OK, so lets talk etymology. Did panties, the garment and panties the word come on the scene before women wore pants?
Hmm, it appears there was a different type of “panty” at first: panty - Wiktionary, the free dictionary
OK, that’s interesting.
I still think the the vague or even visceral reaction mentioned in the OP is due to a conflict with feminist sensibilities and clothing that has a diminutive, girlish name.
“Undies” and “underpants” seem like things only little kids would wear so I would never refer to an adult’s underwear that way.
“Panties” doesn’t bother me, and I suppose I would use it if there was need for more specificity than the term “underwear” affords.
In my house there are a few words that are absolutely not allowed to be uttered. “Panties” is one of them.
Here is the complete list, each one more offensive than the last:
Coke Classic
twine
abacot (it’s an ancient hat but in our household it represents all that is evil in the world)
door (for obvious reasons)
panties (I can barely bring myself to type this word, ugh)
I find this hard to believe. And by “hard” I mean erect penis.
The blogist makes a good point about older men using the word “panties” when talking to a younger woman or girl. That’s definitely cringe-worthy.
I’d never use the word with my daughter for example.
Beat me to it. 
Brilliant movie.
So I’ll ask again, what do you want a 48y/o male use to call them?
“Underpants”? Anyway, why would a 48-year-old male be talking about the undergarments of a young girl who isn’t his daughter?
I’ve realised that’s the heart of the discomfort I have with the word: “panties” are something a prepubescent girl might wear, so hearing non-family-member grown-ups talk about them in any context is, as WOOKINPANUB said on the first page, “vaguely child molestery”.
Is this a serious question? Are you seriously unable to think of any situations in which a 48 year old male would need to talk about the undergarments of a young girl who isn’t his daughter?
Let’s see, without even trying I immediately think of a merchant, doctor, principle, teacher, relative, camp counselor, etc. and various legit situations.
It could just be context but some women are offended by “I want to tear your panties off with my teeth, you dirty cum-slut.”
Chalk another one up for those who are surprised to learn that some people don’t like the word “panties.” It’d be one thing to me if you called them “underwear” or “knickers” or something else, but to say you dislike the word “panties” in particular is new to me.
I think I’ll continue to say “panties” for females’ and “underwear” for men’s.
BTW: 
To a non-American male, panties seems, well, so American.
The offensiveness rating for me is zero. It’s just a word.
I generally refer to women’s underwear as ‘knickers’. Shops usually use the term ‘brief’ or ‘briefs’, e.g. ‘a black lace bikini brief’ or ‘full-size briefs’. ‘Knickers’ are (strangely) not the same as ‘French Knickers’, which are a particular style of lingerie: lacy shorts.
However, my impression is that some non-American women find the word ‘knickers’ to carry the same childish connotations as some American women do with ‘panties’. Possibly from the schooldays when boorish boys would yell “Show us your knickers” at the girls on the sports field.
Maybe they still do.
Anyway, I’d always assumed that ‘panties’ was the standard term in America, and ‘knickers’ predominantly used in the U.K.
It’s child molestery for a man to talk about girls’ underwear? what about the SCOTUS example linked to in the OP?
Not that I myself go around talking about kiddie BVDs but I kind of hate this attitude. The whole idea that something as innocuous as saying “panties” is child molestery.
Using it in a sexual context like BomTek does above sounds child molestery to me, because (to me) panties are not something adult women wear. “I want to tear your panties off with my teeth” sounds like he’s looking forward to removing some ten-year-old girl’s Barbie underpants.
Really? Might that say more about your personal sexuality than that of al “adult women”?
I’ve been to lingerie stores with my GF, there has never been any doubt among us that they are selling panties. I agree the shopping experience here is somewhat eroticized, but that gets to my point earlier - that maybe the dissonance expressed in the OP is between
- the sexualization/eroticization surrounding the garment itself,
- the diminutive word which reminds people of girls,
- the feminist position (maybe explicitly internalized in this era, and hooray for that) that “I am an adult woman, not a little girl, and I should be treated that way”.
Maybe those three things are the source of the conflict?
I think that’s what I was trying to say? The word “panties” is coded as something that young girls wear, and therefore when it’s used in a sexual context, it can creep some people out.
My GF is not a young girl though. She buys what they are selling. In fact, they don’t sell stuff for young girls there at all, but they do sell panties.
I am saying the issue is internal dissonance in some women, perhaps such as yourself, not anything inherent in the word itself, or anyone else’s relationship with or use of the word.
ETA: What I further mean is, that something that makes you think of 10 year old girls as you said is internal to you and arises form your experiences somehow. It may not be uncommon apparently for others to have similar issues, but they are inernal personal issues nonetheless. Not universal and not external.
Well, it is obviously a fairly common dissonance among people such as the OP’s sister-in-law, the authors of several blog posts linked in this thread, and many posters. I’m not sure what you mean by “the issue is internal dissonance in some women, perhaps such as yourself, not anything inherent in the word itself”. Words are meaningless without cultural context, and the word “panties” has overlapping codes of sexualisation and young girls. I’m not sure what “anything in the word itself” would be!
Me either.
But the cultural context is split - these women seen themselves as women, not little girls. That is fine, it is exhibited in lots of ways. If it is real problem, perhaps they can see a counselor to help reconcile the issue.
Really, it is not the panties or the garment that is the real matter here.