General

What do you think of girls who want to be thin?

Honestly my tastes don't matter here. Her body, her choice. If she wants to be thin and is pursuing that goal then more power to her.
5 years

What do you think of girls who want to be thin?

The shear majority of women naturally end up gaining weight purely due to natural hormonal changes and their genetic propensity to store fat. They have a far lower TDEE and hormonal panel (increased estrogen levels) that all naturally lead to weight gain as they age.

So seeing skinny fit women, especially at middle age, is still impressive and striking (though it's not my cup of tea). I know the discipline and sacrifice it takes to maintain that and fight the inevitable tide of aging and genetics.

IMO men should naturally evolve their tastes in favor for larger women as they age. There's something almost prepubescent about a 95lb girl....that should start to feel less appealing. It's like a 40yr old man still seeking out porn videos of little 18yr old girls....that should naturally start to feel creepy at some point in your life. Your tastes should evolve.
5 years