I would believe that It is similar to breeding large and small dog species. The giant would always have to be the female.

And then maybe an engorgio charm or two lol

Only works for wizards and giantesses: Ejaculate on the ground, have the giantess stand over the semen, Wingardium Leviosa on the semen.

Engorgio!

Hagrid seemed to think that it was possible for the Giant to be the father, since he asked Madame Maxime which of her parents was the giants. I would rather not speculate about the logistics of how it works, because gross.

It is gross. Giants are pretty ugly too. I guess it’s the inside that counts but they are also very violent but whatever lol.

I'm gonna answer this in the least icky way I can think of... Magic. The only way it could possibly work without damaging the human is magic and that is the brilliance of this series, nothing is impossible!