Because their society has a completely different attitude towards sex than ours. And because they have a genetic predisposition towards engaging in sexual activity in times of high stress and Satouru was there when Shun wasn't.
It was something we learned from the false minoshiro early on. It was one of the first things they tried in order to make it possible for a society of cantus users to exist without self-destructing, a make love not war kind of thing, but it wasn't enough and they had to go further resulting in death feedback.
2
u/[deleted] Mar 09 '13
Wait, I'm confused. I'm pretty sure Saki loved Shun but why did she have sex with satouru then?