Found via The Corner an official statement from the Thompson Campaign:

In an interview with CNN today, former Senator Fred Thompson’s position on constitutional amendments concerning gay marriage was unclear.

Thompson believes that states should be able to adopt their own laws on marriage consistent with the views of their citizens.

He does not believe that one state should be able to impose its marriage laws on other states, or that activist judges should construe the constitution to require that.

If necessary, he would support a constitutional amendment prohibiting states from imposing their laws on marriage on other states.

Fred Thompson does not support a constitutional amendment to ban gay marriage.

As Hugh Hewitt points out this means Romney is the only top-tier Republican who supports a gay-marriage amendment.