Nobody forces these women to pose like that - they choose to do so to make more money. If anyone should be offended it should be the men who aren't afforded the same opportunities.
It's true but I'm not sure that them not being forced is necessarily key to the sexism argument. I think the culture we have about women tends to assume it's going to happen which is a powerful influence on some women thinking it's normal to do this.
I enjoy looking at attractive women as much as anyone but I find it harder to justify when I think of my own daughter looking at the pictures or god forbid posing in them later in life. I don't want her growing up thinking that is what being a female is about. It's hard enough with all the stupid pop videos where 90% of women dress and act like porn stars knowing full well most of their sales come from little girls and boys.
Just because some women agree to do it doesn't make it a good idea. There were slaves in the US that used to boss and punish other slaves.