Screw having kids, that will ruin your life. Good luck with any career success with little nose miners running around. Good luck getting any sleep. Good luck having sex with her anymore, since she'll be too tired from the kids. You won't be her first priority anymore, either. The kids will be. Have fun with that!
If you mean marriage ceremony, then I don't believe it's required at all to have children. Declaring a child as "illegitimate" because his/her parents haven't signed a legal document is utterly ridiculous. Nobody can be illegitimate. Besides, not like a marriage certificate means you love somebody more or you'll be a better parent.