Why does Fallout 3 have sexual themes?

Despite what most people believe, Fallout 3, just like Fallout 1 and 2, have sexual undertones within them. This is an part of the reality of the wastelands, which in several ways address possible situations that an post-apocalypse will probably have, including sexual themes. However, Fallout 3 has an appropriate age rating of mature, and therefore warns you of this in advance.