User blog comment:B214/Shin DxD 1 discussion/@comment-36663391-20180825085059/@comment-31137559-20180825163442

Issei will only be able to achieve his Dragon God form when he resonates with the 12 Heavenly Breasts and has had sex with all of them. And then he'll only be able to activate the form with the 12 Heavenly Breasts present.

So he'll never become Dragon God-level because Issei will never lose his virginity. XD