It is something that when done right can be amazing, and can create a whole body orgasm. It can feel great.
But it is also a part I reserve for my lover only, and isn't available when working as it takes a lot of trust, and often feels better when drunk, and can't drink, and drive to visits. Along with some other psychological reasons it isn't available for clients, and is for my lover alone.
Besides it needs to be handled correctly, and porn gives a false idea on how to do it. It takes a lot more then just trying to stick it in, which I have seen clients try to do, asking only in the moment they want to try if they could. Assuming it is a hole to be treated like the front, is a good way to make even a good visit, go bad in a split second. Especially when the girl has already made it clear it is not available.
And would add that the responces here are very different than who often contact SPs, as I would say likely 70% of potential clients seem to ask if it is available even when the ad says No Anal.