[Sagittarius]

Finally something I can explain from personal experience.

First off,it depends on your bottom. If they are tense or a virgin they tend to clamp down alot more and experience more pain. This is why foreplay and fingering prior,with or without lube is VERY important otherwise you can cause a anal fissures or tears which increase the risk of STD's. And yes virgins who want to be bottoms,it will hurt. Enemas help though. They loosen the walls of the anal cavity and clean you out at the same time keeping your partner from being obstructed.

If they are relaxed then it won't take much foreplay before you can slide in without much resistance,but yes it still hurts,just not as much. Enemas are again a good idea no matter how often you do it. Just be warned Enemas CAN cause permanent damage if overused or if they are too big.

As Rob said,it does give you the feeling of being surrounded by flesh,I have however found that ever person is different on the Dampness scale. Some are actually wet while others are barely at all. I assume it has something to do with body shape and diet.

As to pacing your thrusts. That has entirely to do with your partner. Men experience it different then women as the goal of Men bottoming is to have the "P-Spot" or prostate rubbed up against,thus sometimes you have to pull out more to re-aim to try and find where to aim. If you can't find it,that normally means you need to change position. While human anatomy teaches you everything in everyone is in the same place. That is not true with the P-spot. It sometimes a deep penetration in the "Diaper" position to find it or a shallow one "Doggie" style. I personally know where my "P-Spot" is because I'm a switch and it takes a deep penetrations in diaper to hit it dead on for me. While with my Ex it was about half way in in "Scissor" position.
So yeah. Hope that explains it. The sensation is more or less being sucked in and out by a damp-fleshy vaccum.