this thread has made great reading, all sorts of comments and thoughts here. So I gotta add something! I'm 43, never wanted kids until I met the wife (spent my life partying!) and even then was unsure (like SFB if I remember rightly)
Now have 2 kids, aged 1 & 2 years and one of each. Don't want more. But they are the best thing ever, and, you will only truly understand the joy (and yes stress, lack of sleep and frustration) they bring once you have them. I am now healthier than I ever was (less partying more biking) I'm not saying have / don't have kids, it's all down to choice but maybe don't be so keen to say "I'm not having any" until you are too old to anyway