There was actually a study that showed female rats are more attracted to (read: had sex with) male rats with very different immune systems. The way she senses this is by the pheromone (smell) he gives off and as a consequence, her offspring have stronger immune systems.
In other words, this study suggests at least to some degree we don't even choose who we have sex with. Haha.
The purpose of my comment was to point out that humans have a (sophisticated) brain, something critical that you had overlooked, and you still seem to have missed that.
You argued that "who we have sex with is a choice we make by thinking about it, not something pre-encoded by our genes." I was trying to suggest maybe the opposite is true to some extent by sharing that study regarding the rats with you. Female rats unconsciously have sex with male rats whose pheromones smell like their immune systems are very different, in other words, who they select as mates is something pre-encoded in their genes.
Are you arguing that because we have brains we can 'override' what our genes would otherwise have us do?
In other words, this study suggests at least to some degree we don't even choose who we have sex with. Haha.