The line is do not involve non consenting others in your sexual experience. Any reputable BDSM club takes that extremely seriously and their members care about other people and consent.
Those busy blurring lines want them blurred because involving non consenting others is an active part of the thrill. This is the case in many paraphilias.
Most of those paraphilias are not equal opportunity in who provides the thrill: the non consenting being involved - who at this point are being used and exploited, and have become victims of someone else's choices and behaviours - are in massive majority women and children.
The 'aww they're just having fun' (and naice people don't have boundaries) thing is all very well, but in the case of all the paraphilias, you are enabling those who are intentionally setting out to use other people as props in their sex experiences in order to be able to congratulate yourself for being all progressive and kind and broad minded in case some just happen to be doing it innocently.
Hint: abusers don't come with a t shirt announcing 'I'm not a nice person'. And many abusers are fantastic at wide eyed innocence and 'I'm just...' excuses. It's all a part and parcel of it.
There are boundaries on freedoms for all because some will use freedoms to exploit and abuse others. And costumes that are an active part of the fetish scene? Yes. These should be restricted, because other people have a right not to be made use of as a walking sex aid.
Do call me any names you feel like calling me for having boundaries, I'm not bothered. I'm not enabling this kind of behaviour and treatment of others, it's not ok, it's not consenting, it's not 'kind'. And providing non reciprocal 'kindness' in situations like this doesn't make you a better person, it makes you a gullible twit.