The Fly I think has a more general theme of the effects of physical entropy on sapient objects.

We all are subject to metamorphosis and disintegration.

Brundlefly is suffering from the aging process as much as disease.

The pregnancy test does introduce certain venereal aspects to his condition but sexual reproduction is another form of metamorphosis and decay.

What comes out the other end isn't us, it's from us.

Wether you see that as a fantastic new variation or a degraded copy is a matter of perspective.