(b) The think that tight jeans are immodest.
(c) They don't care about their appearance and loose jeans are more comfortable.
I realize I won't convince group (c) of anything, so I am only going to address (a) and (b). Let's start with group (a)...
No matter how fat you are, or how badly your ass is shaped, loose jeans make it look worse. Tight jeans might not make your ass or legs look good, but loose jeans will make you look like a man.
As for the group concerned with modesty: you need a reality check. Tight jeans are not provocative; they are normal. Despite what your parents probably raised you to believe, modesty is not something objective. What looks "appropriate" or "slutty" or "conservative" changes significantly depending on two things: context and social norms.
If you were to walk down the street of your city in a bikini in the middle of winter it would shock people and look slutty. Any man who saw it would be turned on. But on the beach, bikinis are normal. Men are much less turned on by them because they are expected. They don't look slutty at all. This is what I mean by context.
Here is the thing: modesty isn't a function of the quantity of skin you show, or the tightness of the clothes you wear; it is defined by how much your outfit makes men (or people in general) think that sex with you is imminent. The more you incite in men the thought that they can bang you easily, the less modest your outfit is. This is why short dresses are so much more sexual than pants. Men see you in a short dress and part of their subconscious recognizes that your vagina is essentially exposed (i.e. from the bottom). Sex seems much more accessible and immediate because there is only one thin layer of clothing covering the most sexually intimate part of your body.
sexual proximity - the criterion for immodesty - is entirely relative to social norms and context. The man on the beach doesn't think sex is imminent when he sees you wearing a bikini, because every woman on the beach is wearing in a bikini, and he knows it is for swimming or sunbathing. But in 1910, if a man saw a woman on the beach in a bikini, he'd be sure that she was a woman of loose morals - and he'd probably be right.
Anyway, the point here is that wearing tight jeans in everyday American life is about as shocking as wearing a bikini on the beach in 2014. And wearing loose jeans in everyday American life is far more similar than you think to wearing a Victorian bathing costume to a Las Vegas pool party. Perhaps more importantly, when you use your clothes rather than your behavior to be modest, you are far more similar than you think to the girls who use their clothes rather than their behavior to get sexual attention. Don't be so sure that you are more intelligent just because you made the mistake about modesty rather than sexuality.
Is there an opposite extreme, where tight jeans start to look like a bikini on a Victorian-era beach (i.e. slutty)? Yeah, probably, but that's not the point - there are always excesses on both extremes. Today I happen to be calling out the conservatives.
So remember: wearing loose jeans doesn't avoid the wrong kind of attention; it just makes you look terrible.
1. "The Wrong Kind of Attention"
2. How to Get "The Right Kind of Attention"
3. Don't Wear Sneakers
4. The Most Important Time to Dress Well