The questions are posed at those very ideas. Why do people associate "realistic" games as mature?

I am posing these questions all around the very concept of maturity itself, the idea of an individual's development versus the idea an individual's identity as "mature".

the people that generally raise this kind of argument, ironically are generally more elitist than the people they criticise

but regardless "mature" refers to the content of the game whether than be violence, sex, etc etc etc we generally try to keep younger people away from these themes and that is the reason for the distinction

Rule of thumb says that anything that calls itself mature is most likely not mature.

This is true.  They're often very hypocritical and lean on heavily biased generalizations in order to paint all M-rated games in a bad light.  It's every bit as childish as the behavior they're complaining about.

Real maturity would be when you play whatever you want without the need to put down what others like.

NO I want to be controlling a lot more video game sex than we're currently being given

yup i absolutely agree

on many occasions i've seen these people resort to calling war games like CoD or GTA or whatever immature because of the greater violent or sexual content and in doing so they propagate the very behavior they claim is objectionable

so i've come to the conclusion that their problem isn't really immaturiy being spouted by individuals, its simply disdain for other people who may have differing tastes and obviously it works in both directions not just from their side, but regardless of the side its equally immature

Adults also play games - they just tend to use different toys

Gaming is not a sign of maturity, or lack thereof.

Violence, sex and gore are just visual themes in most video games, but how they are handled by them most often actually lacks maturity. Dark themes in general tend to be taken more seriously by the developers and thus being also presented in more of a mature fashion to the players.

Violence and gore are often presented in such an over-the-top fashion it's hard for me to take anything even halfway seriously and just looks silly to me. Sex is often presented in a way to titilate 12 year olds at best, and the way to get there is often ridiculous (giving gifts to them to make them fall in love with you, for instance)

Games which truly tackle mature themes in a mature way are rare, I really wish there would be more of them.

I do agree to the reason as to why the games are rated as mature though. The imagery is just not really suitable to most under a certain age. However, I do think a single age gate is not the right way. There should be a description on what's actually inside the game (like sex, violence, alcohol, drugs and so on) and how far these themes are pushed. From  there, one should be able to judge if the game is truly for oneself or not

Games with the mature rating tend to have a mature themed storyline and nature compared to games that don't. The witcher 3. Deus Ex. Fallout. The last of us. Dragon age.

The mature rating in and of itself doesn't make a game mature or not but it's definitely a good identifier to lead ya to a game with mature themes such as politics, convoluted storylines, diving more into the relationships between characters, a wide range of emotions. You won't experience this type of stuff playing spyro the dragon, mario kart, mario tennis, splatoon, arms. etc in general but of course, there is some exceptions to that.

What's more mature, being a good citizen in Animal Crossing or killing everyone in the most brutal way in GTA?

I get what you're trying to say. Lots of kids seem to think they're mature if they play violent games (this partly explains the popularity of GTA and COD).
A lot of people who aren't really into gaming seem to think this too, and they also often believe that cartoony games are just for kids (even though that's usually not the case).

This pretty much summarises it: