Legality of "Parental Gaurantee" Form for MA Apartment renters over 21?

Okay, so myself and my other 2 apartment mates are above the age of 21. We already signed the lease for our apartment, and paid first and last month, through the realtor agency that was in charge of the apartment. Now our landlord is claiming that we did not pay last month (thankfully, we have a receipt so this isn't so much an issue as an annoyance), and also stating that we need to sign a "Parental Gaurantee" form. There was no mention of this in the lease we signed. And we are all over 21. Is this legal, do we HAVE to do this?