They still don't. That's why he has the tattoo on his groin because he is practicing to take the full length of Caius inside...
Actually during one of the first chapters ( towards the beggining not really first) during some fruit incident the effects of it make it so they do go all the way in.
Just swift through the fruit arch. It's a short scene but it's still there!
If you're pertaining on chapters around 14-25 then I didn't see them doing it all the way in. The MC just said that it wasn't hurting him because of the fruit but MC still hasn't been able to take it all the way in. If you can, can you give the chapter reference you're talking about (if chapters 14-25 is not what you're talking about.)
GUYS HELP ME!!! WHAT CHAPTER DO THEY GO ALL THE WAY IN?? AS IN THE FULL LENGTH OF CAIUS PP HEHEH