Shrimps skipper Mark Hughes, sent off for a second yellow card early in the second half, was penalised for a foul on Terry Gornell in first-half stoppage time allowing Matt Richards to open the scoring from a free-kick.
Goals after the break from Gornell and Byron Harrison saw the Robins coast to a 3-0 victory but Bentley felt the decision to award a free-kick for the first goal was pivotal.
He said: "I don't think it was a foul for their first goal and our skipper was booked for it, but he tried to get out of the way of Gornell. But these things happen.
"It's massively disappointing and hard to take because we came here with the game plan and we started brightly.
"We know they were under pressure, with only one home win, and the crowd was muted.
"We had a good early opportunity and Padraig Amond will be disappointed he didn't score, but we looked threatening and played some decent stuff.
"One moment cost us and in my opinion it was a foul on Mark Hughes, but we then defended the free-kick poorly and everyone left it for everyone else.
"We were always up against it then and the sending off made it hard for us, but there were plenty of positives.
"We have players coming back from injuries and we have a big game against Accrington on Saturday, so I've told the lads to get their chins up because they've been magnificent."