I don’t think I would look at it as having sex. I would think more in terms of mating. It is the cow that has the finale say as to whether or not the mating will take place. When her body is ready for an egg to be fertilized the cow will come into heat. The male or bull will sniff at the cows rear often stimulating her to urinate. When the bull sniffs the urine from the cow he will stick his nose in the air and curl his lip in what is known as a flehmen. This gesture concentrates chemical clues as to the cows ovulation cycle in the bulls Vomeronasal or Jacobson’s organ. If the bull senses that the cow may be willing to mate he will lick her side and nuzzle her flanks. If she walks away she’s probably not ready and the bull will try later. If the cow seems interested in mating the bull will rest his head on the cows rump. If the cow is in lordosis, that is assuming the classic ready to mate pose and does not move as the bull rests his head on her rump he will attempt to mount her. The bull covers the back of the cow with his body. His front two legs will be on either side of the cow and his rear legs will remain on the ground behind the cow. This will place the bulls organ of copulation (penis) in proximity to the cows vulva. As the bull is mounting the cow his penis will begin to protrude. Bulls do not go from flaccid to erect as humans do, instead their penises are fibroelastic (semi-ridged) and held (thanks to a sigmoid flexure) curved in an “S” shape inside the body. The bulls penis does contain some erectile tissue but it is mostly muscular action that is responsible for erection and retraction. As the bull finds the cows vulva with his penis he will thrust forward aggressively, some bull’s rear legs will even leave the ground. Thrusting and ejaculation occur almost simultaneously and the bull quickly dismounts.