In this case, they should be able to enter the US on ESTA without issues after we are married, right? We had also been considering getting married in Gibraltar, and if we were marrying outside the US and then traveling there after we were married, I wouldn't be concerned.
I guess my concern is that I know you can't travel to the US on an ESTA with the intent to get married on the trip. Since the online wedding option is only available through the state of Utah, he would essentially be getting married in the US without the "right" type of visa, even though he wouldn't actually be present in the US. It seems like uncharted territory.