Whenever a new game is made. It almost always goes through the initial process of being really fun. However afterwards people start getting bored and tend to either expect updates to the game, or they add their own mods.
Mods are short for modifications and they are done by third party developers for fairly open sourced games. Games such as "Minecraft", "Skyrim", "GTA", and especially the "Sims" series are very easy to mod and so people embrace that.
According to Kotaku, this lead to a mod being developed which implemented teenage pregnancy. Originally, the vanilla Sims 4 game experience does not allow for teenagers to engage in intercourse. However mods did make that happen.
It was first started out by the developer but the mod is now being used quite a bit, enough for people to write articles about it. So why are people using it?
Well the best and most honest answer would most definitely be the fact that it adds more realism to the game. As it is known, teenage pregnancy is a very real and serious issue in the modern world. Considering how The Sims game series are literally real life "SIMulations," it is no surprise that a modder would want to add more realism to the game.
Even Teen Vogue did a similar article and they mentions various people on instagram using the teenage pregnancy mod and creating various story lines out of it.
There was one story about a teenage couple who had an unplanned pregnancy. The girl is overjoyed to be a mother but the guy becomes worried and leaves here. This is just one of the various storylines that are popping up in the taboo of Sims.
Either way, whether it is allowed or not. This taboo phenomenon has spread across the Sims player and it added an even more realistc experience to the game, one that was not planned.