I would really like to know why most females think this is such a bad thing?
"Most" don't.
"Most" don't care.
"Most" just form their own opinion, if they even see your picture, keep it to themselves, and move on.
I just would like to here a good explanation to why guys shirtless pictures are so bad online?
They aren't, except to a vocal minority.
There's never going to be a time when you don't offend someone simply by existing.
If you post a pic of you and a fish, someone's going to complain.
If you post a pic with you and other people, someone's going to complain.
If you take a selfie, someone's going to complain.
If you take a selfie in the bathroom mirror, someone's going to complain.
They aren't going to be the same people complaining for each case.
Or there will be one or two that are complaining about them all and you will focus on them as a figurehead of a giant group of organized complainers.
From your perspective they are going to look like "most" people.
But they're not.