In cases where the state is providing support services for children, a stepparent isn't required to pay unless ordered by a judge so it doesn't make sense that a fiance would have to pay.
"A stepparent is not required by law to pay a support obligation for stepchildren unless he/she adopts the child in care. However, in accordance with Utah Code Annotated 78-3a-906, the juvenile court judge may order the parents or any other obligated person (e.g. stepparent) to pay child support for the child in care."
http://www.ors.state.ut.us/faq_cic.htm#8
You can ask this question at their policy information office, email below:
Child Support Policy Information
[email protected]