That is not the only thing that changed. The earlier belief that circuмcision prevented a sinful habit was an assumption that was not based on empirical evidence. More recently there have been studies that suggest that circuмcision does not affect the frequency of this sin. So, even for those of us who understand the habit is sinful, it is not a good reason for circuмcision.
But there are many studies which back up the claim that it dramatically reduces incidence of the sin. "Empiricial evidence" (i.e. collected statistics can be manipulated), so one always has to investigate the possible agenda of those behind the study. I'd prefer not to post those here, since they're rather graphic ... but several modern experts concur, based on other studies, that it dramatically reduces the sin and makes it much more difficult by eliminating the way that it's done naturally and requiring certain unnatural means. In other words, it has to be completely deliberate. Since the natural method is eliminated by circuмcision, it's far less likely for a boy to discover it by accident.
So I reject your conclusion that it is "not a good reason for circuмcision" simply because you saw one study (probably engineered by the anti-circuмcision crowd) which backs up your opinion. If a Catholic were to lend credence to the other studies, which contradict yours, then they might indeed have good reason for circuмcision.
With every question, there's always a battle of studies ... depending on who's hellbent on trying to prove what.