I think if they're talking about custody issues, they could have a point. If you had a child and then split up and you wanted to return to the UK and your partner was remaining here, then I don't think you could do that without the permission of your partner (UK and NZ both participate in the Hague Convention I believe).

As a PP said, this is only an issue if you were to split up and if you're planning to have a baby, I'm sure this isn't something you're even thinking of!