I've heard from Mexican friends it's meant more like "bitch" and IMO probably directed only at FIFA at this point

Because it is very often used towards gay men. Not exclusively, but often. It's regional, too. Not all Spanish speaking countries use it that way, and some use it that way but with a more general connotation. You'll get a lot of people defending it based on tradition, or based on how THEY use the word (ignoring how others use it), or based on it having many meanings, or based on racism towards their culture. They'll complain that this is white people who don't speak Spanish telling them what is and isn't offensive in Mexico.

But ask a gay man in Mexico what it means to him when he hears "puto" and it won't be about his mother.

Alright then shut up shit for brains go sit in a corner and suck your own ass dummy

Don't be a prude bitch sissy about it just shut up idiot

In Mexico and in many South American countries, Son of Bitch is only one meaning of the word. Another very popular meaning is "male whore." And the vast majority of male whores are either gay or service gay men. This is happening in Mexico, not in countries where it isn't used that way.

It's so prevalent that when you do a Spanish to English translation on Duck Duck Go, puto translates to "gay." On Google translate puto goes to "fucking," so there is no one definition for it. Puta has 10 different definitions listed, including hooker, slut, prostitute, tramp, and dyke... so it isn't a stretch to see why the masculine form of the word can mean male whore.