There are two reasons and types for/of curvature.
1/ Peyronies- This is a sharp bend in the penis caused by abnormal cell growth. Unfortunately this one seems to get more severe as time goes by and can lead to impotence and pain during sex (but on the bright side can be treated if caught quickly enough)
2/ Just curved- If you have a gradual curve in the penis as I do (up) there is no reason to get treatment unless embarrassed. There are conflicting reasons for this one but I'm pretty sure I know the reason why mine curves. All through my teenage years I wore tighty whities with my dick dressed upwards. Unfortunately I didnt buy new underwear through my growth spurts as I had like 30 pairs. So my penis wasnt able to form correctly (by that I mean its curved, and isnt anything special flaccid) because it was so restricted. When I got erections, my dick was forced to curve as it pressed up against my belly because of the tight fabric. So however many clothed erections I would have had as a teenager meant however long my dick was forced to that position. (This is just a theory of mine, and as far as I know, scientists havent hypothesized this. So please do not take this as fact)
Id be curious to know if you downward curving guys wore tighty whities with your penis dressed down. They havent scientifically proven why a natural curve happens but I have my theory which I came up with while studying sex therapy, and would be interested to find out if im correct.
So yeah, you downward curving guys.... boxers or briefs?
Cheers,
Patrick