Sex was a method of biological reproduction for many species.
On the Homeworld, their connection to history through the anchoring of the thread rendered them sterile. Thus for millions of years the concept of sex, like space, was considered irrelevant . The Great Houses reproduced through the use of breeding engines.
However, shortly before the War this changed and natural childbirth occurred through experimentation with House bio-stock and genetic material from the lesser species. House Xianthellipse started breeding soldiers instead of growing them. Even the 103-form timeships were created through a process akin to sex, with the female timeships "mated" to specially made bull-engines. (PROSE: The Book of the War)