David Clarkson fired Rovers into a sixth-minute lead, driving at a back-pedalling United defence before planting a right-footed shot past George Long from the edge of the box.
The visitors equalised eight minutes after the break when Nick Blackman produced a superb left-footed finish from a narrow angle after running on to Porter's throughball.
And nine minutes later Neil Collins headed down a left-wing cross for the unmarked Chris Porter to convert a simple chance from close range.
League Two Rovers more than held their own in the first half, with Derek Riordan and Fabian Broghammer going close. Ryan Flynn had United's best first-half chances, twice firing wide from good positions.
Danny Wilson's team improved after the break and Porter had a 71st-minute header cleared off the line.