The Basal Question

Okie, as I blog this today, I am in serious doubts about my own capability of being able to have an answer for every question !
I have always been a clear advocate of imparting sex-education impartially to all the kids;But today as my son asked me the first basal question about Why do guys like girls and before I could react , very quickly followed the next : Why do they marry ?
I am still searching for the RIGHT WORDS to frame the reply:)
Answering was never this difficult.I feel at a total loss of words today as I have left the reply hanging in mid-air...All my research on parenting seems so bookish and theoretical.
In Practice it is never clear as to how much is too much because from every answer stems a new question !