One taboo activity that gay and straight people both engage in is fisting. It was considered illegal to even show in porn. The stretching of the vagina and rectum is considered where the pleasure comes from, not so much the moving in and out motions.
It can be a risky activity unless one is gentle and uses lots of lubrication, since the tissues in the areas involved are easily damaged, and special care must be taken.
Some people fist themselves during masturbation, thought they have to be fairly thin and flexible to do so. It’s usually done by another person.
There are plenty of sex toys for fisting enthusiasts, that are molded off of real arms. They are big! I couldn’t believe some of the things I saw online, so there is a market for these things, and this is an interest shared by many. Some even use bottles for their fisting fun. You could get plenty stretched out from using a bottle on yourself for sure.
I have found men from England seem to be into this more than North Americans from calls that I have personally taken where they wanted to discuss this topic.
It’s not for everyone, but there are ones that derive enjoyment from fisting. There are many enthusiast sites online for just about every fetish out there, including this one. So get your lube out, not water based lube since it’s quickly absorbed by the body, and maybe some latex gloves and if you dare, explore the fetish of fisting to see if you like how it feels.
