Some of this is speculation, merely a theory. Frankly I could be completely wrong, but for the sake of simplicity I'll write as if I'm certain LOL
Big cocks are psychologically stimulating and dominating to a woman. Women love big guys - muscular and/or tall - because it makes them feel feminine. This is a raw physical aspect that makes and impression on the mind of the woman you're with.
But at the same time, a large amount of what gets them off is purely mental. I mean, there are videos of women being hypnotised to cum and some of their orgasms are totally wild and screaming. Like the dude has described scenarios and given them commands and they're screaming like they're dying - it's crazy!
Think about guys as well - you can have a wet dream while you're sleeping and dreaming of being with an enthusiastic, curvaceous, busty girl - it feels so real in your mind that you have a physical response (orgasm) to it.
Sex seems to happen in the mind to a large degree. And I think that sometimes the mental can override the physical. For instance, I know of a guy who's short (around 5'5") and quite a lot of women want to bang him. Even women who are taller than him flirt with him openly. It's his demeanor, how he teases girls, his dominance etc. that gets them turned on.
I also think there's a social element that overlaps with the physical aspect. Imagine a guy with a 12" cock. It would possibly earn him a high status in a social hierarchy, much like someone who's 7 ft tall. BUT, and it's a big but (no pun intended, although I do like big butts)...it wouldn't necessarily be physically the most desirable thing for a woman to take inside her. Probably way too big.
So what we can surmise is that having a big dick will make it easier to make an impression on woman's mind in the context of sex, dominance, etc. It's a physical attribute that a girl can directly perceive. It has an automatic effect without any words or actions being said or done. At the same time, a big aspect of sex is mental, and can sometimes override the physical.
Ultimately if you can get a big dick, why not? If it makes you feel better about your body, it's always a good thing, regardless of what anyone else thinks.
Edit 1: To add some context: An intense, satisfying sexual interaction has elements of dominance and submission. David Deida talks about this in The Way of the Superior Man. Polarity needs to be there for attraction to be present - masculine/feminine, dominant/submissive, etc. just like magnets (positive/negative). Dominance can be exerted physically and mentally. Physical attributes - big cock, big muscles, height, etc. But sometimes the mental can override the physical.
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