I have this model:
public class SearchModel
{
[DefaultValue(true)]
public bool IsMale { get; set; }
[DefaultValue(true)]
public bool IsFemale
What will you have? You'll probably end up with a default search and a search that you load from somewhere. Default search requires a default constructor, so make one like Dismissile has already suggested.
If you load the search criteria from elsewhere, then you should probably have some mapping logic.