Why would this couple agree to this?
Because their priest told them that they have to.
Because they have been taught by the SSPX that the Novus Ordo priest is not just a laymen.
These are young people and everything they have learned from the SSPX has led them to where they are today.
Likely they married in the Novus Ordo church because it looks better inside than their humble church that was built by real traditionalist years ago, and
given to the SSPX in exchange for the priests who brought them to where they are today, to believing every reason I've put here.