First off, it doesn't happen until after he marries Eris. Secondly, (from what I remember at least) It's only Linia, but there's no mention of actual fucking, she's just used as a body-pillow. Given that Eris is FUCKING STRONG and clings in her sleep, Linia finds the arrangement a little uncomfortable (broken bones aren't uncommon). All the stuff about Eris being dominant in bed is totally accurate, however.