Penises are often curved to the right, to the left, up, or down. A lot of people think it is due to masturbating with one hand, but it isn't.
When erect, the penis fills with blood. You can kind of picture it like the penis is filled with balloons. Those balloons fill up with blood creating an erection. Not all of those balloons are the exact same length, however. If the balloon on one side of the penis is longer than the balloon on the other, then you will get a curved penis. No big deal and nothing to worry about unless it is causing pain.
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"It is nothing to worry about unless is causes pain during erection or sexual activity. It is normal for a man's penis to curve slightly as they get older. In fact some women say they prefer a curved penis because it gives them greater sexual enjoyment.