An argument is valid if, given true premises, it always and exclusively produces true conclusions. A valid argument in this context might therefore be "given that we wish to maximise social welfare (A) and that allowing gay marriage increases social welfare (B), we should allow gay marriage (C)". A and B really do imply C. Some people contend that the argument is not sound (that is, that its conclusion is false) because at least one of its premises is not true (reflecting reality); I am not aware of anyone who contends that it is invalid.

Jiro is contending that people who oppose gay marriage do not do so because they have valid arguments for doing so; if we were to refute their arguments they would not change their minds. Xe has argued above that people (as a group) did not stop being anti-homosexuality for rational reasons, i.e. because the state of the evidence changed in important ways or because new valid arguments were brought to bear, but rather for irrational reasons, such as old people dying.

The fact that Jiro considers it rational to believe that gay marriage is a good thing, and thus that people's beliefs are now in better accord with an ideal reasoner's beliefs ("are more rational"), does not contradict Jiro's belief that popular opinion changed for reasons other than those that would affect a Bayesian. Eugine_Nier appears to be conflating two senses of "rational".

As RichardKennaway observes, we ought to ask why Jiro believes that we should allow gay marriage. I suspect the answer will be close to "because it increases social welfare", which seems to be a well-founded claim.