"But Penny," you object, "ovuliparous critters need to do it in the water, lest their spermatazoa dry out and die!"
We are talking space gem wizards whose -- fluids -- are all most likely anima. And if they were able to reproduce on Freneskae, a mostly anima-dry environment, what trouble would they have on Gielinor? Other than oops, our females are all dead or reluctant?
Mind, Mah was born from an egg, as were Bik, Ful, Wen, and Jas. We have no clue (yet) if the Elder Gods come in male and female varieties; this whole sexual reproduction thing might just be due to Mah's vivid imagination.
Egg laying space gem wizards? Piece of cake for the inventor of sex itself.
Although, the idea and maybe the implementation are older. I mean... plants... we have wheat and various angiosperms in game, and we enjoy the fruits of their ... labor. Bik the Shaper may know something about it.