No filling the role is fine.
Saying that if someone is filling that role then they must be X is gender stereotype.
eg DH is much more maternal than me, I am the enforcer. Our roles are reversed to the norm, that is fine. The suggestion though that he should be called "mother" and I should be called "father" is ludicrous. The names are linked to sex, they are not and should not be linked to expected stereotypical behaviours. Just because I act more like the stereotype "dad" doesn't mean I'm not their mother. Obviously.
So it's not destroying gender roles at all it is removing the sex component and basing them entirely in stereotypical or perceived behaviours.
Well in this case I expect it's just because the form didn't have enough options but that applies to the wider conversation.