But a much easier argument against sports betting is that it ruins the sports. Players throw. They get good at subtly cheating. The gambling apparatus latches itself to the sport, to the teams and players, the umpires and judges, the sporting organizations. With this much money on the line, it's not a matter of if but when games are thrown, cheated -- the bigger the game, the bigger the incentive. It's even easier now because of the amount of side/parlay betting that is available. It exhausts the spirit of competition.
Sports gambling is diametrically opposed to sport itself.
My uncle gambled away a successful business, a beautiful house, his family, his friends. In my early memory he was a giant who carried me in the ocean, flying just above the breaking waves. Later on, when I was in elementary school, he lived with us for a bit. Some time later he lived in his Buick. He died alone and with nothing.
In my mind, we all should not allow a man to do that.
Banning addictive things isn't as straightforward as people love to believe. Even during the worst theocratic times, you could get alcohol in Saudi Arabia by asking the right people; and Saudi Arabia had way harsher means at its disposal than democratic countries do.
(For the complete picture, my grandpa drank himself to death at 57 and even though he used to have a good income, on the order of 3x as much as an average Czechoslovak worker of that time, he left almost nothing behind. All "liquefied". Other people were able to build family houses for their kids with less money.)