The Galaxy had taken a 2-1 lead in the 81st minute on tally by Michael Stephens, but United (3-19-6), which was officially eliminated from playoff contention last week, got the equalizer 3 minutes later.
Porter, wide-open at the back post, got his head on a cross from Perry Kitchen and redirected it past goalkeeper Jamie Penedo for the tying score.
Chris Pontius also scored for D.C.
Robbie Keane recorded a goal and an assist for the Galaxy (13-10-5).
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