I feel like screaming whenever I hear this phrase said.
Actually no, not everyone knows what they were getting in to when they started a relationship with a man with a child/children. They may think they do but in reality often not. I certainly didn't.
I went into it with a few very naive pre-conceived notions of how I though things "worked". Such as; the other parent in the equation will be a rational person capable of putting their children's needs first, if it doesn't work out I can walk away, it's only at the weekend - how bad can it be? (yeah, that one makes me what to slap my former self), I love children and don't want to take their mother's role so it'll be fine just being friends, I've come from a background of being a step-child so I know the do's and don'ts, I'm a nice person and all in all a pretty positive person to be in a child's life (IMHO) so I can't see there being any difficulties etc.
Now fast forward a good few years. The step-children live with DH and I full time after being removed from their mother for their own safety. They have very minimal contact with her, which obviously has caused emotional problems. But their mother still attempts to turn the children against us at every opportunity she has, bringing massive complications to the children's emotional health. We have a child of our own. I love DSC.
So no matter how hard it is, walking away from them isn't an option for me.
I have parental responsibility for all the DSC (complicated story but it was deemed necessary), treat them like my own, love them like my own and I'm the closest thing to a normal "mother" role they have. But still if I dare complain about a difficult time with any of them, or a situation made more awkward by not being a biological parent I get this phrase thrown carelessly at me.
Rant over, can you tell it's a bit of a bug-bear?