Most often the offspring died during childbirth, due to the large size of the infants, but those who lived carried genetics from the father, which gave rise to the legend that these hominoids created the human race, which is simply not true. The giant hominoids from the 12th Planet did not genetically engineer humans. They toyed with genetics, and thus the rumor has started. These giant hominoids were experimenting with genetics just as humans are now eagerly embracing such activity, as they were slightly ahead of humans on the evolutionary scale. Thus when they toyed with the idea of what genetics could bring, doodling with the possible merging of animal and human species, this was taken literally by the people who are discovering these doodles now.
Humans doodle, making pictures of eagles with lightning bolts in their talons.
These were representations of a fascination with the possibility of what genetic manipulation could create. Mankind was created not by these hominoids, but by others who did the genetic engineering, and this erroneous myth should be put to rest. As with any group of entities, all have an agenda. Over the eons, literally thousands of alien types have visited Earth, having one agenda or another. Some agendas are conquest, though these are in the minority.