In the wild, the term is most often used without the space as soyboy but there's a user by that name, so I was blocked from creating a node with that title. It's the latest insult by the alt-right and the reprobate deplorables of the_donald on Reddit to describe liberal or left-leaning men.
As to why that's an insult, allow me to take a few moments to really break it down.
Support for Donald Trump can be understood, in part, as a desire by young men to be some kind of 1950s macho stereotype. Back when men had good jobs and provided for families, and women wanted to marry them and sleep with them. Men had responsibilities, oil under their fingernails, and life was good. Donald Trump spoke a lot about that in the campaign of 2016, and it stuck. Coal miners would get jobs back. Why coal? It's dirty, gives the people who mine it black lung and God knows what else. But it's a dangerous job that is seen as one of the more macho things on the planet to do. Getting coal jobs back was less about winding back the clock on technological progress and more towards this weird romantic notion of "real American men" who drive heavy duty pickup trucks and don't type on computers.
Hillary Clinton, on the other hand, literally said that men were unnecessary. Didn't want their vote, didn't need them, the future belonged to women and all those old white men who ruined this country would get their comeuppance with that combo breaker in a pantsuit. Naturally it was a surprise to her that she actually needed their votes, and now sits in any of a number of Costcos signing a ghostwritten book about why it's everyone else's fault she lost.
Meanwhile, the young guys who supported Trump realized that they were not representative entirely of their gender and age. There were a large and vocal number of young men whiteknighting and virtue signalling going on about how yeah, "I'm with her", men suck, right girls? But not me, I'm one of the good ones. Two of the most notorious ones were two young men who supported Bernie Sanders caught on video arguing with members of the alt-right at some kind of rally. The most oft-quoted meme from that was from one of the men meaning to say "you're a fucking white male" and inadvertently saying "you're fucking a white male". The names given to the unnamed men were "Carl the Cuck" and "AIDS Skrillex". Scrawny, stringily longhaired, and whiny. Everything the assembled rumbling young alt-right hated.
There's a bit of an urban myth that soy is estrogenic and acts like estrogen in the human body. There's a further myth that it's thrown into just about every processed food to deliberately feminize men. The urban legend of Asian men having smaller genitals is also cited, and blamed on soy. "Hey, give a boy in essence birth control pills in infancy, wonder why he's hung like a mouse?"
Naturally, the fact that tofu and other soy foods are stereotypically eaten by vegans, vegetarians, hippies, left liberal Californians and other "girly" men led to the sobriquet "soy boy". One who doesn't ruggedly hunt or eat steak and beef, but instead sips mineral water and gently crunches on some tofu and carrot sticks while whining about global warming. One who doesn't hit the gym, but goes jogging. And has a tiny worthless little dick he's compensating for. You get the picture.
It speaks volumes to the psychology of a young frustrated population of American men and why the alt-right has such a siren song for them.