The Blues centre-back was shown a straight red card by referee Anthony Taylor 63 minutes into the game for being the last man when bringing down Bluebirds striker Kenny Miller on the edge of the box.
Manager Chris Hughton immediately substituted striker Nikola Zigic for centre-back Pablo Ibanez and his mistake five minutes later led to Miller scoring what proved to be the winning goal for the home side.
"My first reaction was to look at Curtis and Curtis jumped up straight away from the incident and didn't feel anything because he touched the ball," Blues boss Hughton said after the 1-0 defeat.
"I think there was enough doubt on the incident that you don't give a red card.
"Stephen Caldwell was in close proximity and I can't sit here and say there's definitely no incident, but there's enough doubt that you can't give a red card.
"The game very much hinged on two instances. Of course the sending off, but also I thought in the first half we coped very well with a very real threat in Miller and I was confident we could cope with it in the second half again.
"The sending-off does change things but we can't make the kind of mistakes that we made and expect not to get punished.
"We have once again lost a game that I didn't think we deserved to lose."