BTW you don't have to go inside anus/rectum to stimulate the prostate. It sits underneath (interior to) the perineum aka taint which is the bulbous area between scrotum and anus. It's actually the part of the erectile tissue of the penis which is attached to the body.
As the penis grows erect so too the perineum grows and becomes more
bulbous which is the meaning of the word perineum.
One of the hottest BJs I got early on was from a guy who knew this. He had one hand wrapped around the base of my erection. The other hand cupped my balls gently in the palm of his hand with thumb and last two fingers but the index and middle finger extended straight back and rested against my perineum.
As he licked, sucked and stroked my head in his lips his two fingers moved ever so slightly against my perineum. He wasn't even moving them. They moved a little just from the rest of his motion. That slight pressure of just two fingers resting against my perineum transferred to my prostate and drove me wild. I never came so fast from a blow job.
I've taught this trick to many men and women who've blown me. I've also used it to test how sensitive a guy is to perineal stimulation and how sensitive his prostate might be. I've made a few guys--especially younger men--come just by reaching in their underwear and pressing four fingers against their taints or rubbing it a few times.
You can experiment with how much pressure and motion to apply. Some like more, some less. No penetration of a stinky unclean butt required.
Here's a pic of a guy with a huge perineum between his balls and anus. I might try three fingers against that! With props to
@Theo0152 for posting it in Dudes Ass.