mg, "You must go and I must bide" does not indicate why "I" have to "bide."   If you don't know the history of Weatherly's lyrics, why could it not be Danny's mother or other close woman singing that he, not she, must go (to battle)?   I don't see/hear anything in the lyrics that makes the gender of the singer clear.   

And, Bonzo, I've heard many "girl songs" sung by men and "boy songs" sung by women (e.g., Joan Baez) without sounding silly at all.   But there are quite a few that really don't come across effectively when sung in the first person by someone of the other gender.