People who hate Nintendo see the company as a childish or "gay" company that is somehow a personal attack on their masculinity or perhaps forces them to face their own insecurities
There is truth to this. Nintendo appeals to kids because their games have bright colourful graphics and inoffensive content.
Nintendo also appeals to adults because adults just want to play fun and engaging unpretentious games and Nintendo generally make the best video games in the world - with a level of design and craft which can only be appreciated by an educated adult mind.
The grey area is the teens and young men, who struggle with identity and maturity. They shun anything which may appear "child-like" because of their own insecurities, while holding on to a bafflingly hilarious belief that video games are a medium capable of delivering deep, insightful musings on the human condition, naunced storytelling and non-cringeworthy voice acting. They will quickly latch onto the most infantile manifestations of "mature" content to validate their own sense of adulthood.
Oooh it has boobies and violence and swearing.
Ooooh its dark and gritty... like my soul.
I am a mature gamer playing mature games.
It doesn't matter to them that these games play like shit or have terrible basic game design.
A quick look at any Best Games Ever list and - without fail - the Top 20 will always be completely saturated with Nintendo titles. The fact that Nintendo keeps their own games exclusive to their own systems also enrages them.