The Sims 4 can only be purchased by gamers above the age of 18 in Russia because it features same-sex relationships.
The rating was announced through the life sim's official Russian Twitter account (via Ars Technica):
The law in question is called "On Protecting Children from Information Harmful to Their Health and Development." It was originally intended to prevent minors from seeing explicit material such as violence, self-harm or drug abuse. However, last year, it was amended to restrict information "[aimed] at causing minors to form non-traditional sexual predispositions, notions of attractiveness of non-traditional sexual relationships, distorted ideas about the equal social value of traditional and non-traditional sexual relationships, or imposing information about non-traditional sexual relationships which raises interest in such relationships insofar as these acts do not amount to a criminal offence."
Characters in The Sims 4 can form same-sex relationships so, according to this law, it must be kept out of the hands of minors. After all, who knows what will happen if they see two virtual characters of the same virtual gender paired off. It might make them think one of these dangerous thoughts:
EA hasn't commented on the rating yet. I imagine they're not too pleased with the situation, though. EA was named one of the best places to work in America by the Human Rights Campaign because of their support of lesbian, gay, bisexual and transgender employees. It seems unlikely that they'd remove same-sex relationships from the game in the Russian version of the game so the rating will probably stand.
The Sims 4 will launch this fall on PC and Mac.