Let me start by saying I strongly disagree with the popular conceptualization of brain lateralization. There are very few actions that are strictly governed by one half of the brain or the other, and even those often change if you are left-handed.
Can you be a little more specific about what part of sex you want to localize? The physical act of in-and-out is governed by your primary motor cortex, which lies anterior to (in front of) the central sulcus. The sensations you feel pass through the thalamus in the brain stem, which routes them throughout your cortex to areas responsible for vision (assuming your eyes are open), olfaction, somatosensation, audition, proprioception, and any other senses that are engaged. Each of these has a specific pathway and arrives at a particular place in the cortex. So which part of sex are you curious about?