There is a discussion about parental rights and how denying marriage would deny a 17 year old parent the same rights as an 18 year old married parent because of how marriage is tied up in all kinds of things. Of course they could address it by an amendment that grants the rights without needing the marriage, but instead they would rather just promote abusive situatuations instead.
Plus they are trying to increase the number of teen pregnancies through abortion bans, so making it more likely that people end up as underage parents.