On second thought, I guess you could get away without a tablix if your dataset contains only one row of information, or is filtered to just one, as it seems might be the case for your report. However, that means that the text boxes have no "default" scope or dataset context.

In this context, you will need a hybrid of your first attempt at the Lookup and a my expression suggestion, something like this:

=Lookup(First(Fields!Gender.Value, "RfsDataSet"), Fields!Gender.Value, Fields!GenderDesc.Value, "GenderDataSet")