Aredwood: Simple. Just make it a requirement that to income split. You have to be either married / in a civil union. Or have kids with your partner. Easy to answer yes / no to those questions.
Even that would exclude many de-facto couples who have no desire to marry or have civil union. For example myself and my partner have been living together for >10 years. Four kids between us but none together. Information that is easily verifiable by IRD, the electoral commission, land transport, MoH ....