To clarify further, when I say the Democratic Congress would not allow the marriage issue to come up, I don’t mean that Congress has formal power to block consideration of the issue. On the contrary, if the DC government votes to legalize same-sex marriage (ssm is strongly favored by officials in DC) Congress would have to deal with the issue, either allowing the law to stand or vetoing it.
But as the article I linked notes, the DC government will consult with congressional Democrats before forcing the issue, and I assume that Nancy Pelosi will urge DC to drop the issue. After all, it’s a political hot-potato, dangerous to Democrats nationally. Yet it is theoretically possible that DC’s government might insist on passing a same-sex marriage bill anyway, thereby forcing Congress to act. In that case, the Democratic Congress would be placed in a very awkward situation indeed.