LOL, love that! I will definitely remember the cake prostitute line next time I'm feeling that way.
But mostly wanted to say, Way to go!! That's awesome that you stood for what's right and didn't cave to the pressure of your customer. Maybe THAT will get them thinking! That's great especially that she still wants to order, so you must have handled it very gracefully. 
I just had to put my 2 cents in. First of all good fro you for being honest. Now if you don't mind I'm going to climb up on my soap box... This "Doctor" sshould not be in practice. Period. 
This is flat out descrimination. 
Why is it ok to make fun of the overweight? They would have never asked you to make a cake that made fun of someone's race or sexual preference. Fat is the last safe predjudice. I personally am sick of it. And as a woman when your having a baby it is the most vulnerable you will ever be!! The last thing you want to worry about is if your doctor is disgusted by you!! Doctors take an oath and they are supposed to treat every patient the same. One last thought...Why would anyone want to reward this jerk with a cake?? I have zero respect for him and his staff. Ok all done now. Way to go on you decision!!!
Way to stay true to yourself - hugs for you.
It is disappointing how shallow some can be, not to mention to ewww factor of such a cake - who would eat any?!? Cake should be savored and loved.
Kudoz for you also watching out for your sons and keeping your rep protected.
Oh and loved the prostitute line btw.
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