Bartleby thinks it’s from mart, the Irish Gaelic word for “heifer”.
Actually, it has. 
Roomie’s cat would frequently have it off with her own mother when younger, and my fag hag’s cat (male) is totally into her other (male) cat.
Well for god’s sake, do tell! Or reply over here http://boards.straightdope.com/sdmb/showthread.php?t=356161
I’m interested!