User blog comment:B214/Question & Answers 5/@comment-26468670-20150725020442/@comment-24098614-20150801035544

I think the reason why she can do this sort of thing with Issei because the Room is in another Dimension isolated from the system of Heaven of sorts making it very hard to for the system to locate and find out what Irina is doing this is my guess about that Special Room.

Irina out of all the girls had the toughest deal of all because nothing personal against Issei I don't trust him keeping himself without lewd thoughts and desires remember he is the mountain of libido as he expressed himself in the earliest chapters of Dxd, and to actually create a child with the other method takes a control of lust and libido that Issei can't control even as much as he has improved from what he was.

Michael saved Irina and Issei a ton of trouble by doing this because doing it by the other method is impossible because I don't think that Issei can keep himself with pure thoughts while doing this act it is beyond him but this is my opinion about the matter.