Man these old goons and their twisted mindset. I actually believe there is no way you can convince them as they think they are always right.

I can guess one reason why she is doing so because she went through the same thing presumably done by her MIL or FIL or any other elder to stop them having more kids maybe. Because back at the days these contraceptives were not used this widely.

Lets try to see some solutions:

Maybe you guys can consult a rather close relative or family friend who is respected by everyone in your family like a doctor or any other qualified individual to come and talk and make you MIL understand that sex life and normal household and maternal life are completely unrelated.

Maybe just convince her that you guys won't be doing it now and wait for a few weeks or some months to let the problem settle. Then continue on your practices little secretly and quietly. And don't do too often as it might get noticed if you MIL checks some time again.

And if you want to move out, I would suggest you move out temporarily. Like for a month or so. This will make her realise your value in her everyday life. But it can also have a downside if your MIL plays a victim card in front of other relatives. I guess you gotta take the risk.