There's no reason to doubt the ability of Ainur to reproduce while they inhabited physical bodies. In fact, full incarnation doesn't even seem to be requisite: we're told that reproduction was one of the activities that strengthened the bond between
fea and
hroa. The Valar may be a special case, but it seems more likely that they were merely prohibited by some
axan than peculiarly incapable.
Here's more regarding
ealar, incarnation, and some about Ungoliante in particular:
Ëalar and Incarnation.