If you’re actually working out, nobody cares about what you look like and comfort is more important than any other consideration.

If you’re using the gym as a place to meet people, nobody you would actually want to meet cares about lines in your clothing.

In other words, the lines you’re concerned with have been put there by advertisers wanting to sell you more stuff.

This is probably not the answer you’re looking for but I’d thought I’d mention it anyways. Have you considered women’s boxershorts? Those are more comfortable when working out and the lines they leave are less pronounced and at a more flattering location.

Since you aren’t getting any recommendations from anyone that’s tried both, I’m gonna give some second hand anecdotal info. If that’s not useful for you, just skip the rest.

Back in my gym days, and through dating women that worked out, strings are generally comfortable enough once you get used to them. They tend to ride up less than something like tangas because they don’t have to overcome the glutes in the first place.

This means that, while you will initially notice the string in the gluteal fold, they don’t really get any more “up there” by being active. I can’t promise you that working out in string undies will be comfortable for you, but the women I’ve known that did, never had any comfort issues. Undies riding up is largely because they’re stretched over our glutes, and as we move, they get pulled up and in. With strings, they’re already in there, so what little pulling they do experience tends to be less noticeable.

However, they do still sometimes show lines if the outer clothing is thin or tight enough, but that will be in a much less visible place, and whatever top you’re wearing should cover that just fine.

Now, as far as accumulating sweat, everything you wear while sweating is going to accumulate some. The only way to avoid that is to use materials that repel water entirely, which just causes other issues with the sweat.

But the key is that the string is in a place where the sweat is going to be less of an issue. While the buttcrack/gluteal fold does sweat, and sweat runs through there from other places, it’s not going to pool there for long, and the string isn’t bulky enough to absorb much in the first place. So you end up with less noticeable sweat lines than some other types of undies, and it doesn’t trap moisture in the fold, or around the genitals.

So they’re a good choice for avoiding visible lines, as well as reducing the print of sweat through leggings or other form fitting clothes.

Cotton is actually where you want to be. It does absorb sweat, but that’s a good thing. When it sucks it up, it also distributes it. That means it evaporates off faster. It’s also a very breathable material, so your parts aren’t going to stay as damp while sweating. Damp cracks and genitals are not a good thing. That’s how jock itch loves to set up shop. Heat + moisture + darkness = fungus paradise.

Cotton undies are pretty much the gold standard for working out, imo. That’s for men and women. You’ll stay drier, get less chafing; the undies dry out faster as you change, so there’s less funk in the gym bag too. There’s some arguments to be made about the blended fiber “wicking” fabrics, but good luck finding those in form factors that won’t print through leggings.

Now, like I said, string undies are a different feel. Some people never get used to them, and hate them. And I say people because men do have string options if they go looking for them. But simple cotton ones aren’t expensive, so there’s not much risk involved in trying some.

Now, I will also add that while strings are less visible through clothes, it isn’t like that’s necessarily what you want. You didn’t specify what kind of noticeability you are worried about. String undies do very little to hide the glutes, even less than tangas. So while you lose the lines, it may make the shape of your glutes more noticeable in other ways. Imo, it isn’t much, and anyone already looking at your backside isn’t going to be paying more or less attention to what undies you have on than they would in any given type of undies.

What you might get less of with strings under leggings is attention from fashion police. If you’re more concerned with being ogled, it’s a bit of a coin flip which way will be more noticed by the kind of folks that are going to give more than a glance. Lines vs no lines, it really depends on how the ogler thinks, so it’s not something that’s entirely possible to negate.

I’d guesstimate that guys will end up paying less attention without lines on average unless they’re just horndogs that will eyeball any woman just because. The horndogs, you really can’t predict as much, they’ll find something rude to think (or say, though most of them aren’t stupid enough to say it on the gym floor at least) either way. But for guys that just take a look and move on, the lack of lines gives the brain less texture to pay attention to, so they don’t linger as long. I have no idea what kind of locker room talk women have, so I can’t guess in that regard.

Again, this is all based off of second hand info. Women discussing the options with or around me, men talking about such things (fwiw, most locker rooms, the horndogs get shut down pretty fast. Serious gym rats don’t put up with their shit.)

Well, the fabric stuff is partially my own experience, plus the general advice given by the medical community about preventing issues from sweating heavily. Cotton really is the best overall option for workout clothing. Nothing else has that mix of properties where it wicks sweat away, doesn’t retain it much, but still evaporates it at the right rate to keep you cool in roughly the same way bare skin and sweating would. Even the best blends don’t improve on cotton, they shift the balance towards a given factor.

If you are having problems with cotton maybe look at a material better at wicking. I wear wool or viscose. Cotton is anti-wicking, making you hot and sticky at first then cold and clammy as it cools down.