Dunne said: "I have seen it again and it's an appalling decision that's cost us.
"Magnus was about one yard away and he has no reaction time to make a deliberate decision to block that ball - it's impossible for him to get away from it.
"We were booed off at half-time and we gave the fans nothing to cheer about in the first half.
"I told my players that and we got stick that was warranted and that if they wanted to get the crowd on their side, they had to show character and commitment.
"Our fans were magnificent in the second half and we gave them something to cheer about.
"I said to the guys at half-time that we could get it back in the second half and we did.
"We showed lots of courage and to have it taken away by a really poor decision like that is gut-wrenching."
The Blades struck twice in the space of two first-half minutes to take a 2-0 lead.
Maguire headed in Tony McMahon's free-kick in the 10th minute before Porter closed down Magnus Okuonghae's pass and delivered a cross-shot which flicked in off Tom Eastman.
Matt Hill struck the crossbar for the visitors, but Colchester reduced the deficit three minutes after half-time through Macauley Bonne, after Clinton Morrison had flicked on Luke Garbutt's delivery.
And Garbutt drove in a superb 25-yard shot from Brian Wilson's throw to equalise just after the hour-mark before Porter's controversial penalty sent the Blades through.
- Sports & Recreation
- Magnus Okuonghae
- Chris Porter
- Harry Maguire