You have to pick:

By what means?
Magic? Me? Random guys?
As long as it's not the middle one, I'm fine with this.

I'm thinking its magic but you would show up as the father in the paternity test.

Is this like an aura of pregnancy or a one time thing?

Permanent aura

If it's by magic, then would they still get pregnant if they physically can't?

would be a fun party trick :P

awkward when you visit your family

Like Zoidberg?

It occurs to me that this doesn't limit the effect to *human* females.

Those paternity tests could potentially become extremely embarrassing. And many interesting hybrids would ensue.

I think (and hope) it means human females.

Is this a one-time thing, or is it that every time you vomit, it's marbles?

I think it's a one time thing