Mr Bush briefly brought up the topic, unprompted, while raising money in Iowa for a Republican congressional candidate, a day after the New Jersey Supreme Court decided that same-sex couples must be given the same rights as married people.
The court left it up to the state's Legislature to decide whether to extend those rights under the structure of marriage, civil unions or something else.
The president said the ruling "raises doubts about the institution of marriage".
"I believe marriage is a union between a man and a woman," he said. "I believe it's a sacred institution that is critical to the health of our society and the well-being of families, and it must be defended."
The US leader did not say how it must be defended. But he has advocated a federal ban on gay marriage.
Earlier this year, a proposed constitutional amendment to ban same-sex marriage failed to win the needed two-thirds support in both the Senate and House.
Gay marriage is legal only in Massachusetts.
Other US states, including Vermont and California, have laws permitting civil unions or domestic partnerships that offer virtually all the benefits of gay marriage, except the name.
New Jersey and some other states have domestic partnership laws that offer fewer marriage benefits.
Twenty US states have passed constitutional amendments aimed at blocking gay marriage, mostly by large margins.
Eight more states - Arizona, Colorado, Idaho, South Carolina, South Dakota, Tennessee, Virginia and Wisconsin - will be voting on November 7 on constitutional amendments that would ban same-sex marriages and, in most cases, civil unions as well.