I would never joke about something I find distasteful.
As she told me, Morrigan needed a Warden's child to absorb the taint of the Archdemon when it died. That would result in a being with the powers of an old god, but none of the taint of the Darkspawn. I do not know why she could not ask Riordan, but she told your companion that it needed to be you.
Apparently she convinced you, because she loved you and could not bear to watch you die. Or to die herself and leave you alone. She said it was a choice between this and a lifetime of being alone.