FWIW, I reckon you need to turn that 'starting a family' argument around, maybe take a wee bit more of a positive approach this time, e.g. "this is something I'd like to get out of my system, it'll help me settle down and I'll feel more ready to start a family". And when finances are the main concern, I find it helps to put all the numbers into a spreadsheet (or at least on paper), so that I can prove to the missus (and myself!) that the purchase I'm proposing is in fact affordable
You could maybe even take her out somewhere nice for a meal before Sunday and talk it over...? Then if you're not wearing curry/chow mein/spaghetti by the time the bill arrives, you could be onto a winner
Anyway, best of luck mate. Have patience - you know burds, sometimes they just like to test us by making these things difficult, eh?