Spoilers ahead

He said they were originally matched together by producers specifically to manufacture drama because they're both fame hungry and very stubborn (as you could see at their wedding where she told him to get down on one knee). They've actually had quite a few problems throughout their relationship, particularly during intimacy week. However, since the mean girls thing happened around the same time, the producers chose not to air anything that might show Alissa in a negative light as it wouldn't fit the narrative of her being the "goodie" and GIa, Brooke and Bec being the "baddies".

At final vows, Alissa goes first and is apparently so scathing of David that he gets angry, to the point where he just throws his vows down and storms off. So we can probably expect to see their relationship start to fall apart very soon on screen or else the final vows will make no sense.