"I once loved a sailor" leaves it pretty open actually...
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I don't much like the idea of changing the gender words generally. I wouldn't exclude it absolutely. But often keeping the same words changes the song in an interesting way when the gender of the singer is the "unexpected" one. For example "Sometimes it's hard to be a woman". Or "Girls don't marry.

And sometimes changes the gender helps [people hear a familiar song in a fresh way, regardless of who is singing. "She who would valiant be" is an example - I remember someone writing about how they used it that way for a daughter's christening.

But I don't thing it makes sense to think you always have to change a song to match your own sex as a singer in an automatic way. "Handsome Johnny" instead of "Pretty Polly", or "Brenda on the Moor"...