I love it, but I know how niche it is and so I avoid sharing stuff that contains it.
I know this is /d/, but like it's been said: a very small niche.http://holykleenexbatman.deviantart.com/art/Egg-Relay-Part-3-207104507
– Four part prose, but the cock birth/egg transferral is covered in the third part (which I have linked).
There's another story on deviantart with mpreg cock birth, but I always have the hardest time finding it since it's not tagged very well. It involves an underage mpreg - sex with an alien on an island settlement of humans and aliens - and a ritualized process for birth. The writing could be better, but I still enjoy it.
(You will need to have the web site set to show mature submissions. This doesn't require logging in, though.)
– This manhwa comes with a few warnings: rape, shota, incest (technically). I abstract those out, so I am unbothered by them, but I can see how it may put off others.
(You need to have a free account at g.e-hentai.org and log in to it before visiting exhentai. Otherwise, you'll end up at a web page with an image of a sad panda.)