Republican Representative David Schultheis called it a privilege to have been the first.
Legislators who sign the pledge formally state their intent to fight for laws that prohibit gay marriages, and ensure marriage remains a union between a man and a woman.
Critics say the pledge goes much further.
Senate Democratic leader Joan Fitz-Gerald of Golden says she's been in a heterosexual marriage for 31 years, but won't sign the pledge calling it outrageous.
The pledge also targets gay or straight couples who live together.
The Washington, D.C. based Family Research Council is promoting the pledge.
Viewers with disabilities can get assistance accessing this station's FCC Public Inspection File by contacting the station with the information listed below. Questions or concerns relating to the accessibility of the FCC's online public file system should be directed to the FCC at 888-225-5322, 888-835-5322 (TTY), or email@example.com.