I think it's fine with traditional songs, which are more focused on the story, but modern songs, which tend to be more personal, I wouldn't do. Sara Carter sang songs from a boy's viewpoint. Roscoe Holcomb sang songs from a girl's viewpoint. It doesn't really matter. And if a person's still uncomfortable singing a song from the point of view of the opposite sex, they can just add a verse to the beginning that involves walking out and hearing a girl/boy saying the rest of the song. You see that in a lot of songs like The Frozen Logger, Johnny I Hardly Knew Ya, I Never Will Marry, etc.