This pledge to support a Constitutional amendment banning same sex marriage is meaningless posturing. There is no way, even if every Republican voted for the amendment, that it would receive the tneeded wo-thirds majority in both houses of Congress. Even if this were to happen by some miracle, you would need three fourths of the state legislatures to ratify it. That means 38 states. Of the 11 Northeastern states, only Pennsylvania would possibly pass it. Of the five West Coast states, only Alaska would ratify. Even not counting Minnesota or other liberal states in flyover country, there are 14 states that will not ratify.
One reason Trump is doing so well in the polls is that he does not engage in meaningless posturing in supporting impractical solutions.