If the job hasn't changed and each of the two people can still do what's required of them equally well, why should one earn less than the other? The man shouldn't be earning more because he's got more years at the job. He should have been promoted in to a higher paying and more responsible role. That's what experience should count towards. If he hasn't been promoted and he's still doing the same work then there's no reason for him to be earning more than the woman.

People should be paid for doing a particular job, not for being more experienced, or older, or 'better'. Promote the people who are better and more experienced.

This is the beauty of the Basecamp pay model. What the job pays increases even when someone isn't doing it, so when they come back to work they'll be on the higher salary because the market values that job more.