User blog comment:B214/High School DxD EX/@comment-24053170-20170109193427/@comment-24053170-20170109215042

Actually tatsumi can't change back. his original human form was destroyed after he allowed Incursio/Tyrant to fully evolve in order to defeat the emperor and the ultimate teigu.

This was confirmed by both Akame (when it first happened) and the last chapter of the manga.

The only thing human thing left of tatsumi's is his brain/consciousness.

So yes, the sex with Mine is basically impossible but since they say it happened were going to have to accept it as the truth until the author says otherwise.... ^^;;;

Though it might be possible if he can reduce his physical size.