This seems to be a modern dilemma that's becoming increasingly more common. It's (mostly) single women with children who suffer because unless they have a very good job/income, they end up claiming Tax Credits to basically survive (like I have to.)
The drawback with this is if she then finds a dp and somewhere down the line they decide they would like live together, she looses her Tax Credits (not saying she should stay get them by the way) and ends up financially worse off and then has to rely on her dp, who isn't the children's father, and it's a lot of pressure for everyone.
That's why I think it's important that if you find a dp, he needs to fully accept that you come as a package with dependent dp and not as a separate entity. If you choose to live together and it means you will loose your Tax Credits then it has to be a 50/50 split on bills. No partner can expect to just rock up in someone's home, make them loose a vast chunk of income then expect not to foot that shortfall. The woman in this situation should NEVER end up worse off.
I would urge any woman in this situation to be fully transparent to her dp about what will happen if they live together with regards to finances. I don't live with my dp yet but he knows when we do live together I will loose my Tax Credits and won't be willing to be worse off.
If you look at it in the long run it's not forever. Kids aren't little forever so if your dp can see past the next few years and help out then you'll know he's a keeper.
If the man thinks it's unfair that he is paying in part for someone else's kids whilst making the mother financially worse off by him being there, he's not a keeper.