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ABC News' Michael Falcone and Russell Goldman report:

Newt Gingrich, who has married three times, promised personal fidelity going forward and provided a lengthy written response to an Iowa social conservative group's so-called "Marriage Vow." But he did not sign the pledge.

"I also pledge to uphold the institution of marriage through personal fidelity to my spouse and respect for the marital bonds of others," Gingrich wrote in response to The Family Leader's request for him to sign the 14-point pledge.

Gingrich also promised to "enforce the Defense of Marriage Act," to support a "federal constitutional amendment defining marriage as the union of one man and one woman" and to "oppose any judicial, bureaucratic, or legislative effort to define marriage in any manner other than as between one man and one woman," among other things.

The group's leader, Bob Vander Plaats, an influential conservative in the state, said mambers of the organization were "pleased that Speaker Gingrich has affirmed our pledge and are thankful we have on record his statements regarding DOMA, support of a federal marriage amendment, defending the unborn, pledging fidelity to his spouse, defending religious liberty and freedom, supporting sound pro-family economic issues, and defending the right of the people to rule themselves."

While Gingrich affirmed the principles in the Family Leader's pledge, he declined to sign it.

"Speaker Gingrich did not sign the pledge, but provided his written response affirming the principles laid out in the Marriage Vow," according to a statement from the group.

Family Leader's "Marriage Vow - A Declaration of Dependence upon Marriage and Family," equates same-sex marriage with bigamy and polygamy and calls on candidates to promise to be faithful to their spouses.

The two-page pledge includes a "Declaration of Dependence on Marriage and Family" that blames several factors for the deterioration of traditional marriage including "quickie divorce" and unmarried couples living together. The pledge also describes homosexuality as a choice and not genetic.

As the Iowa group points out, Michele Bachmann, Rick Perry, and Rick Santorum are the only candidates who have officially signed the pledge. So far, the other GOP candidates have declined.