Fewer people than you might think, these days, will just accept “it’s for charity” as a reason to put their hands in their pockets, and this is not entirely due to the fact that fewer people have any spare change or indeed spare time to give. There have often been requests for contributions to charity in my own little corner of the world: donations for a raffle to be held at some signing events and some markets, donations of stories to fundraising anthologies… There’s nothing inherently wrong with asking, of course, but you should be prepared to answer questions and accept polite refusals for a range of reasons.
There are some charities that some people fundamentally object to and will never wish to support (you’ve undoubtedly got a few examples of your own). It might be that people think or indeed know Charity A is atrociously run or an outright grift designed to line the pockets and feed the ego of the founder. For those of us involved in the adult industry or living non-mainstream lifestyles, there are ‘charities’ which we know are actively toxic towards groups they consider less than human, or whose approach to a particular issue is more harmful than helpful to those it claims to be saving. People may be disinclined to support animal charities when there are human beings in need, or they may object to medical charities where funds raised may contribute towards experiments on animals.
If you are running sex/kink/erotica events, there’s another reason to tread carefully about including ‘for charity’ stuff in your shows: your preferred charity may refuse your offered assistance (and, if you have chosen particularly carelessly and they are a charity run by a bunch of utter wingnuts, they may publicly humiliate you for your good intentions: I know of one organisation whose donation was rudely rejected by quite a large, mainstream charity on the grounds that they did not want to be seen to profit from ‘immoral activities’.)
This is not to say you should bin any idea of including some sort of charity element in events, just think it through… Like these guys did.