User blog comment:B214/Volume 23 discussion blog/@comment-26821288-20170205085556/@comment-9595671-20170208140701

Going by how some of the other girls have acted in the many interrupted harem situations, I don't think any of them are willing to give Rias the first time, they all want it so... maybe Akeno snuck it from Rias right under her nose for all we know ^^;

And do we even know if Akeno and the other reincarnated Devils don't get the Devil's lower birthrates? That would be a good question to have answered, especially since it kinda makes sense for them to have better birthrates due to the situation post-Great War.