I too have a silly theory, based on the following observations:

1) Kyrie has a typical "male" body shape (rectangular, not rounded).
2) Vaarsuvius, when talking to Quarr about the need to rescue his children, uses the phrase "adopted progeny" - so they are not his own children.

Which makes me think that K and V can't have children on their own, and why would that be?

Because V is male, too!

Of course, it's not conclusive evidence, but I think it's likely. Did anybody have the same theory before? (Probably)