I've tried to take a cock up my arse a couple of times but it was so painful I had stop before the head was in.I've dreamed about a man fucking me many times but the reality was different from the fantasy
You have to prepare yourself first by getting some experience with a dildo. Use plenty of lube and go slow - there might be a little pain but it should stop. If it doesn’t you are doing something wrong. Your sphincter muscle has to get used to being penetrated.