Soliek it has nothing to do with it being an all womens class. The problem is the actual moves. People will be soon get used to doing these moves that are not fit for a Bas Yisroel. It’s like you hear somone speaking a very vulgar language the first time your are horrified, by the 4th time that you will hear it, it won’t be so bad. Same thing over here-something that is not so Tzniyus in the beginning will soon be ok in the end but it is really Not ok!!
BTW the same thing goes for the Break dancing that men do at weddings as well-it might be cool and fun to do but if you know where that is coming from-We just got used to it so now it is ok. It is Not ok either!!