You have asked some very interesting questions. When I became interested in finding out what it would feel like to be bred, I experienced the same hesitation as you have expressed. I realized eventually that I had to let someone breed me so that I would know one way or the other whether I would want to have that experience again. The first experience was uncomfortable, if not a little painful, and it took a while for me to recover. At the same time, the guy who had penetrated me was a really nice guy, attractive, physically-fit, and expressed concern that he had hurt me. I explained that it had been my choice to have sex and that I had accepted full responsibility for what I had experienced. We became great friends and we had sex many times again. After the first experience, and having fully recovered, I wanted to try again, and this time it was wonderful. He took his time and told me to let him know if what he was doing was hurting me and assured me that he would stop. To my greatest surprise, his cock rubbed against my prostate gland and I had an incredible orgasm, much more powerful than what you would experience masturbating.
So, to answer your questions: If you haven't been bred before, you'll likely experience the penetration as uncomfortable, so having the guy lubricate his cock well will likely make the experience more pleasant for you. If the guy hasn't had sex for a while, and he's really turned on by you, he'll likely have a huge discharge which you may or not feel. I have had several loads released in my butt, one after the other simultaneously, which I felt. I had a feeling of fulness with some of the cum leaking out of my butt. I have, on several occasions, felt my top stop fucking and his cock pulsating as he discharged his cum inside me. It's an incredible experience having the guy fucking your butt at a high speed, then stopping, and holding his cock still deep inside you while discharging his cum. The experience after your top has withdrawn his cock makes you feel that you have done something good for him. You have the feeling of satisfaction that both of you have benefitted from the experience. I love the feeling of keeping my top's cum in my ass. Depending on the amount of cum deposited, you may have the urge to release it in the toilet or, if you don't have the urge, there is the psychological pleasure of having given your top a beautiful experience by making your body available to him for his pleasure. Pushing cum out of your butt is also pleasurable, at least I have found this to be the case.
Yes, to answer your question about how it feels to get fucked, the pleasure increases with repeated experience of being penetrated. The first time you do it, as I said, it can be uncomfortable, but with experience, it can reach the level of ecstasy with the right person. If you feel weird doing it the first time with someone older, perhaps you shouldn't have done it. It takes preparation, good judgment, and good communication, to have a great sexual experience. Hope this answers your questions.
Jamie