Imagine if your cock could sort of talk or think. If my big shiny bell end could be thinking oh god no I dont want to be going inside her rectum today its dark and untidy in there. Imagine its fear as it sees her bum hole coming closer then gets pushed inside going deeper and deeper before its allowed to come up and take a breath. Then just spends the next 30 minutes being constantly pushed down through bum hole and sphincter into the rectum up and down sliding sliding sliding all wet and sloppy inside there then has to explode a load and now its out of breath.
I guess if it didnt really like it then it would just go limp on you in the first place so it wouldnt get chance to go in there.