Your problem is that you are not allowed to shift into a self closing tag and also you get an error because of not piping your "labels", so Jade thinks they are vars.
Please try this:
span(style='display:block;')
div
| Sex:
input(type='radio',id='Male',name='sex',value='Male')
| Male
input(type='radio',id='Female',name='sex',value='Female')
| Female
To produce this HTML: as JSFiddle
<span style="display:block;">
<div>Sex:
<input type="radio" id="Male" name="sex" value="Male"/>Male
<input type="radio" id="Female" name="sex" value="Female"/>Female
</div>
</span>