Men vs Women

There is one question that in my 16, almost 17 years of life that I have never understood. Why is it okay for girls to wear swimsuits out in public, at a pool, or to post pictures of themselves in on social media, but bras are the unforgivable sin?
My favorite comparison I've heard was made a few years back, in the early 2010's, I'm thinking maybe 2012. Back when gay marriage rights were still a big deal. But anyway, the comparison was that "bras and swimsuit are just like gay and straight marriage. Both are the exact same thing, but only one is publicly accepted." Because it's 'different' it's not okay in public, even though in reality it's the same.
And I wouldn't even care that much, except it's only focused towards women.










So between the two girls there is one positive comment. One. I really don't think either of their outfits are that bad. You can't see nipples, and you can't see a vagina so I don't see the problem. If there were going to be any negative comments, I feel like it should be aimed towards Demi's makeup. She's got quite a brown/orange hue to her face and then very pale chest making it very obvious that there is lot of makeup and it's not blended very well.  But now let's take a look at another photo.

Here's Justin Bieber and his naked butt. Well naked everything. And the comments are all positive. I really don't understand this because I as a woman find nothing attractive about a man's butt nor do I want to look at it. I personally am much more offended to see the man's butt than the girls perfectly covering outfits! Then all the comments also are saying for Bieber to turn around, but why? As much as I dislike looking at the man's butt I definitely don't want to see the front either.
So what's the point in picking on the girls that completely covered themselves but then contradicting yourself and telling the man to show everything? And it's not an uncommon thing. This isn't the only time stuff like this happens. It happens almost every time celebrities post pictures in every fandom.  Like for Brendon Urie and TOP many people will comment how they want to see more. For example in Girl/Girls/Boys many people comment on the video for the camera to continue to pan down and such. But then when it comes to Melanie Martinez or Halsey, many people dislike them because "they don't dress appropriately enough for them". Even if that was true, how does their outfits change how you listen to the music?

Honestly I don't think it should matter either way. We all have bodies, we all know what things look like, it's not a big deal. It's not like it's taboo that I, a woman, have boobs. If you see my bra strap, too bad. I guess somebody better call Victoria and tell her that her secret's out.