He said on ABC's "Good Morning America" the same thing is happening with the issue of marriage that happened with gays' service in the military.
Changes in attitudes by military leaders, those in the service and the public allowed the repeal by Congress of the "don't ask, don't tell" policy that will eventually allow gays to serve openly in the military.
Gay marriage is still not legal in most states. President Barack Obama recently said his feelings on the gay marriage issue are evolving, but he still believes in allowing strong civil unions that provide certain protections and legal rights that married couples have.
Obama said he is still wrestling with whether gay couples should have the right to marry, now that the change in the law will allow them to serve openly in combat.
The question came just hours after he signed landmark legislation Wednesday repealing the ban on gays serving openly in the military. The law ends the 17-year-old "don't ask, don't tell" policy that forced gays to hide their sexual orientation or face dismissal.
But in letters to the troops, the four military service chiefs warned that the ban is still in place, and will be for some time to come.
Recommendations to implement the repeal were outlined in a report last month, and now must be formed into concrete regulations. Defense officials say that they still don't know how long it will take before the Pentagon completes its implementation plan and certifies the change will not damage combat readiness. Once certified, the implementation would begin 60 days later.