If you must do this with onclick then I'd suggest:
<input type="button" class="gnM" onclick="$('#gender').val($(this).val())" value="male" />
<input type="button" class="gnM" onclick="$('#gender').val($(this).val())" value="female" />
JS Fiddle demo.
I'd strongly suggest moving from an onclick to a jQuery unobtrusive method, using the click() method:
$('.gnM').click(
    function(){
        var that = $(this);
        $('#gender').val(that.val());
        that.css('background-position','bottom right');
    });
JS Fiddle demo.
To allow for toggling:
$('.gnM').click(
    function(){
        var that = $(this);
        $('#gender').val(that.val());
        that.siblings().removeClass('active');
        that.toggleClass('active');
    });
JS Fiddle demo.
Or, more concisely:
$('.gnM').click(
    function(){
        var that = $(this);
        $('#gender').val(that.val());
        that.toggleClass('active').siblings().removeClass('active');
    });
JS Fiddle demo.
References: