Okay, so there was a dramatic progression. Once upon a time, all of my anal muscles were very tight and constricted. To the point where the slightest attempt at resistance came when the doctor would insert just a finger in my ass for a DRE. Did not appreciate it at all. It always hurt, and there were a few times I thought it would cause me to scream or squeal. And that would have been so embarrassing. Especially when you think about anyone outside in the hall, waiting room or wherever who might be able to hear you. Much less the dominant doctor has brought your submissive self to that point.
With the subsequent female nurse practitioners and med care professionals, their fingers were smaller, and somehow they went in more gently. than the way then-homophobic me thought their counterparts did.
So in the early days of my bisexuality, there was no anal involvement at all. It was all about cock. I had a host who would insist his visitors all shower prior to sexual activity, as it pertained to where his tongue was going. While you stood in the shower (for me, bracing myself on the side bar), he would coat his finger with liquid soap and stick it in my ass and around the perimeter and perineum to clean it, since his tongue planned to be there soon thereafter.
The first time he rimmed me, all of those sensitive neglected nerve endings thrived. I convulsed into a long series of dry orgasms. I would try to get myself to relax, and within a half second, I'd be going nuts again.
Then after the nerve endings became not so neglected anymore, the sensation became less, but still good.
Then there's this other guy I've been playing with for about 5 years. Just found out last week that we both share the same birthday. He often likes experimenting with things in my ass. At first, I couldn't let him get too far before I needed him to stop. He was always respectful of my limits, and over time, my tolerance has been growing. He's now up to inserting a vibrator. I just don't like it hitting the sphincter, but when backed up just a bit, the sensation is really nice. And I like the fact he has me tied down to the bed. Damn, I'm getting hard typing this.