As if you just got kicked in the stomach, the magic jolts you again. You bend down clutching your stomach area, and your tail ducks between your legs. Your stomach rumbles and twists before the pain moves down and settles in your crotch. You feel a balloon-like swelling down there and your heart sinks. "No, not that..." you think, certain you're about to grow an embarassingly obvious udder. You reach up and shut the bathroom door, not wanting any more witnesses.
The swelling continues until the pain of the witch's spell is replaced by the agony of your new-grown flesh straining against your underwear. Knowing there's no way to hold it in, and not wanting to break your pants even more, you take them off and are startled by two large lumps hanging down in a very spacious sack. It dawns on you that they're your balls--but bull-sized.
"At least I'm still male," you mutter. It doesn't comfort you too much, though--you still feel extremely violated that even your manhood is fair game to this unfair witch's hex. Noticing that you don't see your dick yet, you pull your shirt off and notice a long, narrow tube attached to your belly and reaching almost up to your chest: a sheath, covered in solid white fur. You suppose your penis is tucked away inside it, just like a bull's. Your hands come out to curiously pat your new anatomy, and sure enough you can feel a firm, pointed rod through the smooth fur of your sheath. The firmness surprises you, since you're definitely not aroused. You pull down on the sheath, trying get a look at the thing, but it's completely fused to your abdomen.
Nevertheless, you start to feel a tingling down there, as if you are actually about to get an erection. But instead of the swelling feeling of your soft dick slowing filling with blood, you get a loosening feeling deep within your crotch before a pink, javelin-like rod instantly juts out from your new sheath. Surprised and disturbed, you lose whatever arousal you had and in seconds it has retracted back inside.
That's when you remember a tidbit from a lesson in a biology class you once had. A bull's penis doesn't usually get much larger or harder during erection. Instead, the muscles holding it in the sheath relax and let it out.
"Freaky," you say aloud, your first bovine erection still fresh in your mind. You're still disgusted by the thought of a bull's penis, but since you now have one you decide you ought to get a better look at it. You rub your sheath and try relaxing that muscle again. After some experimentation, your cock reemerges even further out this time. Amazed at how thin it is, you wrap one hand around the exposed portion, which sends a blinding jolt of pleasure through your body. You grip tighter, immediately lose control, and start humping into your hand. Almost as soon as you do that, you hear your horns scrape the ceiling as your whole body tenses up, and you start feeling the spurts of your ejaculation. Then, breathing heavily, you release your grip and regain your senses. The loose feeling of your erection subsides, and your sticky pink shaft gets drawn back into your sheath. Your hand's sticky too, but surprisingly it's not much more semen than you could have made with your old equipment. The pointed tip of your penis disappears. The whole affair lasted less than half a minute.
"That's it?" you wonder aloud. You're a little embarassed--you didn't really intend to go that far. Part of you is glad you did though. After all, your little experiment WAS very pleasurable, not to mention informative. It's just that, ... um, ... for something the size of a bull, ... you just expected that there would be, ... well, ... MORE to the whole experience. You sit down on the toilet and reflect on how different sex must be for a bull.
Dejectedly, you realize that's what you are now. A bull. Well, not exactly, but with this tail and these horns you're never going to pass in human society again. And it's safe to say you can't very well have sex with human women or have human babies. You're only hope now is that either changes will reverse (which you're not expecting) or that you finish the transformation and live as a bull (which you're not exactly looking forward to).