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.