Dropdownlist for with Dictionary<string,string> is not working with selected value

I am trying to bind a dropdown in MVC3 in this way.

Model

public static Dictionary<string, string> SexPreference()
{
      var dict = new Dictionary<string, string>();
      dict.Add("Straight", "S");
      dict.Add("Gay", "G");
      dict.Add("Bisexual", "BISEX");
      dict.Add("Bicurious", "BICUR");
      return dict;
}

Controller

ViewBag.SexPreference = MemberHandler.SexPreference();

View

@{
var itemsSexPreference = new SelectList(ViewBag.SexPreference, "Value", "Key", Model.sexpreference);
}

@Html.DropDownListFor(m => m.sexpreference, @itemsSexPreference)

Dropdown is not selecting the selected value, don't know why.