There is no evidence that Cathars were given to practice sodomy.
The accusation probably stems from the observation that the Cathars regarded procreative sex as worse than non-procreative sex. So, Catholic theologians reasoned, Cathars must regard sodomy as being less culpable than conventional sex (true) and they must have practiced the former (false).
This was an effective and persistent accusation. Remember that Cathars were given many names. When they first appeared in Western Europe they were known to have come from the area be know as Bulgaria. They were thus called Bulgres, a word that Church propaganda turned into French Bougre and English Bugger.
Ironically, sodomy has always been widely practiced in the Catholic Church, though never formally condoned. Various church orders were famous for it - Voltaire was particularly fond of ribbing the Jesuits about how widespread it was in their Order. And it was not only practised between Catholic men - anal sex was commonly practiced in Catholic countries between man and wife as a means of contraception.
Since Cathars had no moral objection to other forms of contraception it seems likely that, on average, Cathars would have had less need for recourse to this practice as a means of contraception, so it is possible that they practised sodomy rather less than their Catholic counterparts.