What does it mean to know?

I have started with a new therapist and we have been focusing on the difficulties within my blended family. At some point Tuesday night, after I was complaining that I didn't feel appreciated my therapist said: You knew he had a son when you married him.
I felt bad when my therapist said it to me. This statement feels very discounting and shaming to me. I spent a few hours trying to understand why. What is the assumption behind the statement? That I should have expected all these problems? That I could have been prepared? This was how I defended myself to my therapist. (Notice I said defended, I felt attacked). I tried to argue that there was no way in hell I could have been prepared for this, never having had kids, not expecting my stepson to live with us since he was in Russia and kids rarely leave their moms in Russia. Add on top of that that I didn't expect my stepson to be such a difficult child. But let's be totally honest here. I knew my husband had a kid when we got involved. Did I give it much thought? No. I was head over heels in love and I only thought about how my heart ached at the thought of not being with him. End of story. Was this smart? No. Does criticizing this now help? No. This leads me to some examples I thought of while driving in the car to work yesterday. When someone complains about how bad their husband is, why don't we say: Didn't you know you husband when you married him? It wasn't an arranged marriage, was it? When someone complains about how difficult their teenager is, why don't we say: Hmmmm. You knew he would grow up into a teenager when you decided to have him didn't you? You weren't ignorant of biology, were you? You understood that sex can lead to teenagers didn't you? When someone complains about their boss why don't we say: You had an interview before you took the job, didn't you? You met him, right, you investigated the company? I could go on and on here but I think you get it. We don't say these things to people when they tell us their problems with their boss or kids or husbands BECAUSE IT DOESN'T HELP --- BECAUSE IT JUST DISCOUNTS THEM AND MAKES THEM FEEL STUPID. IT BASICALLY SAYS "YOU MADE YOUR BED NOW YOU SLEEP IN IT." IF THIS KIND OF ADVICE SEEMS HELPFUL TO ANYONE PLEASE RAISE YOUR HAND.