The president told reporters in the White House Roosevelt Room, "The amendment process is addressed in any serious matter of national concern, and the preservation of marriage rises to this level of national importance."
Saying gay marriages would forever change the institution's meaning, the president declared that marriage cannot be separated from its "cultural, religious and natural" roots.
President Bush said his decision is in agreement with the "overwhelming consensus" of Americans, believing recent court decisions to be the work of "activist judges" and not reflective of common opinion.
"Unless action is taken, we can expect more arbitrary court decisions, more litigation, more defiance of the law by local officials -- all of which adds to uncertainty," the president stated.
The president made his decision after the Massachusetts Supreme Judicial Court recently ruled it unconstitutional to ban marriages between same-sex couples. Mr. Bush has also expressed consternation at the roughly 3,000 gay marriages performed at San Francisco's City Hall over the last few days.
The president said states should have the right to outline "legal arrangements other than marriage." His remarks suggest that he would not use the proposed constitutional amendment to ban domestic partnership laws or civil unions.
During the Clinton presidency, the federal government attempted to prevent state-by-state recognition of gay marriage by passing the Defense of Marriage Act, but President Bush warned that courts could strike down the act without the passage of an amendment.
More than two-thirds of states have passed laws or amendments banning same-sex marriage.
The proposed constitutional amendment could take one of two routes to passage. The first would entail a bill passing both houses of Congress with a two-thirds majority in each. After passing both houses, the amendment would require approval from three-fourths of states, through legislature or conventions.
An alternate method would require two-thirds of state legislatures to call a Constitutional Convention, and for the convention to propose an amendment which would then be sent to states.
Democratic National Committee Chairman Terence McAuliffe expressed his organization's opposition to the proposed amendment.
"The Democratic Party is opposed to this amendment," McAuliffe said. "It is wrong to write discrimination into the U.S. Constitution, and it is shameful to use attacks against gay and lesbian families as an election strategy."
He said the president was using the issue to distract voters from other thorny issues, such as health care, jobs and the economy.