I think it could possibly relate to the theory people have talked about here with Diego being an omega originally. Or maybe even Diego being 'made' like that for Shane by Shane's father to create more of them? One thing I'm thinking for sure though is that Shane is having an impact on his body either way. I'm thinking Shane's pheromones are naturally changing his body, or it was already there but Shane is helping his body "work" properly again and it's re-opening that hole. Idk I'm just talkin at this point lol

I didn't even know there was an mpreg tag for this manhwa (love it though) but surprised people didn't bring up chapter 13 when talking about it. Since chapter 13 shows Diego having another hole/entrance inside with liquid coming out of it before Shane goes all the way in.