As a versatile bear, let me give you the real rundown - yes, it hurts - at first.
It takes PRACTICE. It's not something you just slam in, contrary to how the pornos look. The anal entry does expand (regardless if it's male or female), but you have to get past the sphicnter muscle - the ring, if you will. That's the tough part, and that takes PATIENCE. Always best to practice alone before taking on the big guys - fingers, dildo, what have you. And lube - LOTS of lube. Lube is your friend!
You have to find a fella who's patient, and will work his way in slow at first - so that you can accomadate him at YOUR pace. Once you get past that, then you can let your fella buck away, and feel how great it is to have your prostrate gland massaged from the inside out. You can practice with a vibrating dildo to get an idea to see if it makes you feel good. If it doesn't, then it's not for you. If it does, you'll shoot like a geyser.
As for being loose - in a word, KEGEL! Do your kegels! It's a simple exercise of tensing the testicles and butt muscles. The benefits is that you keep your butt nice and tight, and strong too. It has the added beenfits of stronger sphicnter and bladder control, and farther-shooting ejaculations!
Do a wikipedia on it - it's dead simple.
And just to expand on Bbucko said - the emotional aspect DOES play a roll - unless you're into domination/humiliation scences, find a top guy who'll fuck you with pleasure and patience, not with the intent to immasculate you. I learned to be a great top man from being a great bottom bud - see if it works for you.
Have a good fuck, buddy!