Interesting question… but I don’t have an answer for you unfortunately (reg the radio button solution). I think it’s hard to prevent this, the way you set it up.
What you could do (but this requires some coding) is to create 2 checkboxes instead of radio buttons for gender. Make this field required and don’t select a default value.
Then with javascript, create a function that disables the other checkbox (on change), so only 1 option can be selected.
