Moral allowances change with times rather dramatically, and a very relative, so I wouldn't rely on them too much. For example, a few centuries ago it was perfectly normal in Europe to marry 13 year old girls.
If it comes to judging „what a truly tolerant person should do“ you make an absolute normative, hence moral claim.
This strikes me as rather inconsistent and therefore unconvincing.
On being 'confronted' with the question about legalizing genocide. Well, if there ARE people with such ideas, it's not like making the ideas illegal will stop such people from having them. So open discussion about it and democratic voting will have just 2 outcomes: nazis will be either reduced to nothing and their political base will dissolve, or they'll get to power and a genocide will happen. However, I believe that a society that will openly vote for genocide is beyond saving, whether you criminalize some ideas or not.