> It's a very fundamental human (nay, multi-celled organism) instinct to try to pass on our strongest genes.

There's nothing fundamental about it, it's just what we (and our instincts) are implicitly optimized for. I very much doubt that there's a biological basis for the drive to have kids. That involves some quite elaborate concepts which would be difficult to encode in the molecular machinery produced by DNA. A biological basis to seek pleasure, and to find pleasure in sex, though, sure.