User blog comment:B214/Shin DxD 1 discussion/@comment-2A00:23C5:1A8D:9900:8CF3:A504:BAF1:B216-20180702131314/@comment-10330822-20180710075125

I feel like its become a gimmick that Issei is not going to lose his cherry until he marries one of the girls. Since none of them will allow the others to get ahead of them by having sex early. It can't be rationalized who should "go first" since all of them what his first.

It makes sense, because its already been established that the harem needs some kind of schedule in order for it to work. Issei will probably marry them all in order and then have a sleeping schedule with each one after the fact.

Don't worry, if EX is any indication, Issei makes up for lost time after the fact. You don't have to worry about him getting any at all. Because he someone managed to make all his devil wives pregnant despite the fact devils are difficult to get pregnant. Meaning he must have tried quite a lot.