Ahhh, the joy of blended families.
To be honest this is a great betrayal to me.
The betrayal is not to you but to her children. She has denied her children upwards of 17K of financial support that the state has awarded to her for their care.
You knew what you were getting into when you chose to marry her. A blended family often includes (in some way, shape or form) an ex-spouse. Your mistake was counting on his child support payments as supplemental income. If you choose to marry a woman with 3 kids, expect to support them.
Your wife's child support income is not something that you have any control over, even if she does receive it. She can use it, spend it, waste it, gamble it, invest it, give it away or burn it if wants to. The court does not dictate how she spends it, only that she receives it.
Am I wrong in expecting a man to pay his "full" state expected amount of child support?
Yes. You are wrong to expect anything of your wife's XH. He doesn't owe you the time of day. You should however, expect your wife to financially support her children to the best of her means and ability. That includes not only accepting but demanding full child support payments from the kid's father as the court has awarded.
She is essentially writing her ex a check each month for $900 without consideration of myself or the rest of our 7 kids.
I feel for ya. This is really stupid of her and she is cheating her children. It's not like it's money her XH doesn't have. It's just money that he would rather not spend on his children.