I enjoy my partner (female) doing it to me now and again. It's an incredibly intimate, powerful act of trust.
The reason I am mentioning female is erm... there's a difference in the size of her hands compared to most men's hands.
But that area is a muscle, and if you are sufficiently relaxed, lubricated and aroused enough, with a partner who is willing to go slow, working their way up, I can't see the issue.
I have also fisted a woman vaginally and fisted men anally. You can't use enough lube with the latter, and with either one, you can't ever go too slow or too gentle. That would be my main concern in doing it with a random client - if they don't take it slowly enough, get over excited and just, bluntly, ram it in there.
I re-read your message and you mentioned it as a lesbian sex act. Erm... I have engaged in it with 3 partners but I can't say, speaking to many of my friends, it is something many of them do. I just have a try everything twice mentality, and the first time I did it, it just... happened. I had a partner who asked me to do it to her, but I don't enjoy doing it because I am too scared of hurting a woman in that way. As a Domme, I do it ocassionally on men.
TL;DR - lube, lube, lube. Go super slow, gloves are good, you won't cause lasting damage, no we aren't all fisting each other
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