It's a common belief that angels are incapable of reproduction, if that is so then why is there a story about God castrating the angel Samael, he wouldn't need to if angels cannot procreate.
As Mary's answer has already said, angels often are portrayed as capable of procreating. The origin of the Nephilim in Genesis is an example of this explicitly stated in the bible, if briefly. I wanted to add an answer to expand on the idea that angels can't procreate and where this comes from. Modern Christian dogma often takes it for granted that angels are sexless. This seems to derive largely from Matthew 22 where it is said that there will be no sex and marriage in heaven. However, the logic behind it also has to do with the way that our view of spiritual entities have changed over time.
Many modern Christian groups will insist that angels must be genderless because they are "pure spirit", as if spirit is non-material and therefore can't be associated with forms. This isn't really the way that ancient people looked at spirits though. Spirit was "subtle matter". It was something different than the vulgar stuff of the physical world, but it was still stuff. Spiritual entities in the ancient world had bodies, wore clothes, ate food, etc. Even the above mentioned passage in Matthew frames the lack of marriage in heaven as being due to the fact that we will be given new, purified bodies with different sets of needs, not due to the fact that we will be naked souls with no bodies.
Angels in the Old Testament have sex and come over for dinner and get into wrestling matches, and they also manifest as unknowable cosmic horrors that can't be approached or understood and make people fall on their face. There was no contradiction in these perceptions of spiritual beings in the ancient world. A combination of platonizing influences and modern rationalism, however, have resulted in a clean break between the physical and spiritual that makes such fuzzy lines untenable to many modern believers.