shein, as long as they are properly reporting all household income, they are definitely not violating any civil law. Some might argue that there are maras ayin issues associated with obviously-frum couples cohabitating and/or having children without being civilly married, but that is a separate issue.
However, if the couple is not married and c”v one becomes ill, the other partner may not have legal standing to make medical or other decisions on behalf of the ill partner. And if c”v one partner dies, the survivor may likewise encounter legal problems regarding inheritance of the estate (assuming that both parents are listed on any children’s birth certificates, custody of the children would probably not be an issue).