Why did you let yourself get two kids with the woman you love without marrying her?
Marriage is a dream of many ladies. Even the career oriented ladies once in a while think of marriage just for the experience of it all.
You didn't give her that experience and I bet she felt insecure despite having two kids with you as you are not married.
And no matter how many kids you do have, if you are not married, you can't stop her wedding unless the guy he is marrying don't know about the kids and react in your favor.
You could have thought that someday despite how many kids a woman has or how old she is, she won't miss someone who will give her a hand in marriage. This is real life!