What if you don't want kids but end up with someone who has them?

I really liked this 1st person story Kids? I prefer animals in the Guardian family section from a woman who had never wanted kids but then found herself going out with a bloke with three children.  Funny, positive and poignant it points to a dilemma that childfree women can find themselves in. What do you do if you definately don't want to be a mother but end up in a relationship with someone who has them, finding yourself in the role of stepmother?