Mind flayers, also known as illithids, are apparently hermaphroditic creatures and don’t reproduce in the normal sense. They lay eggs from which tadpole-like creatures emerge which are used to make more of their kind through a process called ‘ceremorphosis;. Basically, they capture someone and render them docile through a psionic blast. A newly hatched tadpole is inserted into the victim’s head, usually through the nostril or ear-canal. The tadpole grows and devours the brain of the host, attaching attaching itself to the brain stem and essentially becoming the new brain of the host. Over the course of a week, the host’s body changes form and morphs into a new mind flayer. The new mind flayer does retain a few dim memories associated with its former host, but they’re only vague and seldom have any affect on the new mind flayer. While I can’t find it specifically mentioned out anywhere, I do assume they have genitalia of some kind, but my feeling is that they – and the sex act itself – would be fairly rudimentary, but that’s not specified that I can tell so you could easily change that to suit yourself.