How would st. thomas defend himself (if, indeed, such a defense is possible) against the charge of being “homophobic” (a word that did not exist in his time, but which is fairly common today)? would you find his defense plausible?

Saint Thomas would most likely said that the only reason he conducted the 'homophobic actions' was because it violate the rules of God.
In that time, this kind of defense was perfectly plausible since the Church at that time had a huge influence in the Government. But our time, this type of reason wouldn't be plausible.