It was a day to forget for the Sounders, who had United States star Dempsey dismissed after a video review in the first half.
Already trailing 1-0 to Roland Lamah's 20th-minute opener at Toyota Stadium, Seattle's woes were compounded when VAR stepped in to give Dempsey his marching orders for an apparent elbow on Jacori Hayes eight minutes before the break.
Sounders veteran Dempsey then sarcastically applauded the referee's decision as he saw red in Frisco, Texas.
It was one-way traffic from that point, Dallas – which drew its first match of the campaign – making the most of their numerical advantage in the second half.
Maximiliano Urruti doubled the lead just prior to the hour-mark for his first goal of the season, while Lamah netted his second five minutes later.