User blog comment:B214/Question and Answers 3/@comment-4547267-20140921221703/@comment-24866657-20140921225226

I believe it's more than that, perhaps we will get the explanation of it when vol 19 comes out, but here my theory:

1. At that time Issei already a part of dragon due to his left arm, so somehow his gene will be passed to his child.

2. Chance to have sex with a pervert is much higher than someone like Vali/Kiba, or else why would Xenovia bother with Issei?

3. Xenovia knows that Issei is a good man at heart, when she badmouthed about Asia Issei had told her to stop and apologize, even though he could be killed with a single slash. That's the strength of fool and brave.

4. Xenovia can getaway much easier if it's Issei, if suddenly Issei demands this and that then Xenovia is still strong enough to control the situation.