It's not that the clitoris becomes the penis in some cases; it's that they originate from the same developmental tissue, with genetics (usually the presence or absence of a Y chromosome) triggering them to grow in different ways. In terms of sensitivity and pleasure, the head of the penis is still very sensitive. So someone may get a lot of pleasure from that being stimulated the same way someone gets a lot of pleasure from the clitoris being stimulated (or they may not enjoy it that much; just because people talk about the clitoris as a pleasure point doesn't mean that everyone enjoys it being touched).
Also, the nerve endings in the whole of the penis and the whole of the clitoris are probably about the same, they just aren’t spread out the same ways. Comparing the whole of the penis to the glans of the clitoris, for instance, isn’t really a sound comparison because that glans isn’t the whole of the clitoris but only one small part where a lot of the nerve endings are concentrated, much like with the underside of the glans of the penis.
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