YANBU. Existing children should always be the first priority of all adults involved.
WRT NRP's new partners having to contribute, I do think there is a discrepancy in how it's dealt with by the "system".
I am a single parent. I work full time and receive tax credits both to top up my income and to make a massive contribution to my childcare costs.
If a new partner moved in with me and he worked, his income would be taken into account when assessing my tax credits, tax credits that I get on the basis of having children. My tax credits would be reduced, effectively forcing him to make a financial contribution to children that aren't biologically his. He would have to accept this as part of moving in with me.
If a new partner moved in and (for whatever reason) didn't have a job, I would lose all my childcare related tax credits, because tax credits say that he can care for the children (again, who are not biologically his) while I work. Again, both I and this hypothetical partner would have to accept this as a condition of him moving in.
So, as a RP my new partner's income/circumstances would be considered in respect of the state financial help I receive for my children, and they would have to accept that as part of being in a serious relationship with me. Why then are the income and circumstances of NRP's new partners not considered for CSA purposes?
In my own personal situation, I don't suppose I would want my XP's partner's income to be considered, I don't want her money. BUT, it is extremely frustrating that he can effectively live off her income and not work, meaning he is not required to pay maintenance, and there is nothing I or anyone else can do about it.