Bella is half a Distressed Damsel!Sue and half a Boring!Sue; she's constantly needing to be rescued, remember? -.-
And yeah; it's especially grating when a) angels are involved or b) if you're talking a cross species breeding, for example, centaur and mermaid, which indicates that the Suethor Failed Biology Forever.
As for angels; well, in everything I've read on them, they're supposed to be asexual, or at the very least, to be composed of light particles and therefore unable to breed.