Just wanted to throw out there that you may not get many answers at the moment because the forum is still recovering from a crash - many members are holding back and waiting for things to smooth out. Your thread may disappear as things get fixed - this is an unfortunate side-effect and nothing personal. Just a heads-up!
Personally (married mom here,) I probably wouldn't care which gender my massage therapist was, so long as s/he were doing a great job! But many women are more comfortable being nude (or nearly nude) in the company of other women, versus men. You know your wife, though - is she particularly conservative about her dress around men? Is she insecure about her body? Depending on those answers, if she refuses, it might be more about her than about the answer she thinks she's "supposed" to give.