I was putting on a bright red polo shirt the other day and being a straight white American male it occurred to me that almost nobody of that description ever wears red jeans of similar color. They exist and can certianly be purchased easily on Amazon, but ill bet 99% of said described men dont own a pair. I think they are perceived as being “gay” maybe? But i dont really understand the origin of this, i would wear a shirt of the identical color and no one would think to question my sexuality over it for a second, so why is it different if its on a pair of jeans instead?
It’s not just jeans; men’s casual and dress pants are almost always either dark-colored, or a neutral khaki. Part of it may well be that they require less thought – regardless of the color of your shirt, indigo denim jeans, or a pair of black or khaki pants, will probably “go” reasonably well with the shirt.
This is better suited to IMHO.
Colibri
General Questions Moderator
There’s Nantucket Red khakis and shorts but that’s about it.
We 'merkins don’t wear red pants, should not be trying to wear leather pants and should avoid vests and loosely-wrapped bohemian scarves. Conflicts with our sense of selves rooted in puritans and Cowboys.
Also, most American men*, frankly, have very little fashion sense. They want to wear something that’s comfortable, and requires fairly little thought (and for many of them, I’d wager that their wives select many of their clothes). There’s also probably a tendency towards conformity with their peers.
*- myself included. 
American men have a very circumscribed palette of colors which are “not gay”.
I remember an acquaintance a while back who asked her husband to go clothes shopping with her. He told her just looking at women’s clothing in a dispassionate way, in a department store, made him feel panicky because it felt “gay” to him.
I don’t have any real idea what the terror of doing anything which feels gay is like for American men but since it seems men spend a simply phenomenal amount of energy enforcing this fear amongst themselves at the most impressionable ages, it isn’t surprising that it controls a wide swath of behavior in them.
Most American men can comfortably wear a pink or red shirt without being considered “gay”. I’m wearing a bright red shirt right now, and don’t feel the least bit gay!
Why that doesn’t extend to pants, I don’t know.
Every time I see a guy wearing red pants, dude looks like a clown. I don’t wear red pants on the off chance that I, also, would look like a clown. It’s worked for me so far.
I guess the question would be: Gay or Hipster? 
Poor guy must be color blind is what I’d think if I saw someone wearing red pants.
Speaking only for myself, I don’t wear them because I’ve never seen them for sale. I’m sure I could find them online somewhere, but who wants to shop for jeans online?
My daughters sometimes date/guy freind, Hamza wears every color pants you can imagine. He is very fadishly stylish. He’s 19, I think millennials are much more avant-garde about clothes.
It reminds when one of my brothers grew longish hair in the 70s, my Daddy went ballistic. Now it’s considered okay. Long or short hair on guys is just down to personal preference.
Why is that? It seems like American men have less fashion sense than any other culture. They also seem rather proud of it. Why is that?
Back in the 80s, when all those pastel colors were fashionable, a friend of mine wore a pair of salmon colored jeans to work.
I said, “Chris, I hate to tell you this but those jeans make you look gay.”
After a moment he looked puzzled. “Didn’t you wear a pair the same color last week?”
“Yes I did Chris, but in my case it makes me look like a guy so confident in his sexuality that he can wear salmon colored jeans. In your case, well…”
It hasn’t helped me … I suppose I should ditch the big shoes.
My fashion sense is non-existent, but aren’t colored slacks a thing right now? I see them on guys. It never occurs to me to think ,oh, he must be gay. When I see them I think of his age. Mr.Wrekker would have to be dead to be in any color other than denim or camo.
Most American women don’t wear red jeans either. It’s been a good 20 years since I last saw women’s jeans in red, green, orange, or purple more than once in a blue moon. Do they still exist? Sure. If I visited 20 brick and mortar stores today would I find some in those colors? Probably not.
But why? Red is just as primary of a color as blue is, why is blue “normal” and red virtually unused?
Howard Wolowitz wears red jeans.
I have no fashion sense because I am not interested in having fashion sense. Therefore, I try to wear the most nondescript, unremarkable clothes possible. Red would be right out, for shirts or trousers.
History and inertia.
If cochineal dye was less expensive than indigo back in the day you would be asking why there were no blue jeans available today.