You apparently have been living under a rock because there is a world of difference between someone who is nice and also a guy and someone being a “nice guy”.

Also the posters are red flags not because they reference nerdy things but because they show an unhealthy obsession with fictional characters who, though charismatic, are evil people who justify their violent actions with straw man arguments about power and corruption. The words on the Thanos poster put an especially fine point on that part.