See this pisses me off, it's like everyone with a digital camera thinks they are a wedding photographer people think a big cock makes you a bull.
Guys being a bull is far more about attitude and confidence than cock size.
Agreed with lerotic and ZDong -- the main ingredient is attitude. I'm pretty big, as far as the general population, at 7.5/7.75 x 5/5.25, and there are definitely times & moods were I feel pretty dominant and bull-like, and my wife & I have great sex in those moods. And I definitely fantasize about playing a bull to the female half of various couples I know.
But the fantasies that *really* get me going are cuckold fantasies -- say, some big muscled 8 or 9 x 6+ guy comes and screws my wife's brains out, dropping a big load on her face and making me lick her clean (I'm also 6' and 150, so pretty skinny, and I like the idea of a bigger, stronger guy doing it). And for the couples we know where the guy generally fits this picture -- taller and/or more built than me, it's all cuckold, all the time. Even If I start out fantasizing about such a guy's woman with me in a bull-role, I just gravitate toward imagining him coming in, and showing me who's boss by screwing the brians out of his woman and mine.
Whew. Just thinking about this has me rock hard -- time to go have some fun

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Great thread.