Frontrunners Hillary Clinton and Barack Obama both defended their opposition to same-sex marriage.
All of the Democratic candidates support a federal ban on anti-gay job discrimination, want to repeal the "don't ask, don't tell" policy barring gays from serving openly in the military and support civil unions for same-sex couples.
Most Americans oppose nationwide recognition of same-sex marriage and only two of the Democrats support it - Mike Gravel and Dennis Kucinich.
Mrs Clinton said her stance on marriage was "a personal position".
Mr Obama said that from his perspective, civil union "wouldn't be a lesser thing".
"Semantics may be important to some but what I'm interested in is making sure that those legal rights are available to all."
For the front-runners this was a carefully calculated appearance, but one where there was in fact little to distinguish between them, our correspondent adds.
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