Can a non-paternal father be forced to pay child support in Canada?

My father keeps telling me that it's the law in Canada that a non-paternal father can be forced to pay child support. I can't find this answer anywhere so at this point in time I don't believe him. However I have been in relationships in the past where the girl has a child. I don't want to be stuck paying child support for someone I get serious with just because the kid has come to call me dad. If I were ever in a situation where I needed to pay child support for my own child then that's perfectly fine. Should I be weary of getting serious with a girl who has a child?