"I don't mean to offend but what is the difference to you that the child is genetically yours? why not adopt?"

This might be an unpopular answer, but it makes all the difference in the world. I want my family line to continue; I want my parents and grandparents to see physical features and behavior quirks in their (great-)grandchild that I haven't even noticed.

It's a primal urge and I can't explain it rationally except to say that I'm human and no exception to what we are hard-wired for.