Pixie can get pregnant, yes.
But she doesn’t get a preggo sex scene.
She’s way too small and the scene was a headache to animate.
And also she goes with you everywhere, so in some scenes it wouldn’t make much sense to have a pregnant Pixie flying around you. So her pregnancy is pretty much instant.
Lorewise, it’s that she’s made directly with Artifact energy. So if a regular girl’s pregnancy lasts weeks, Pixie’s lasts a few days.