I haven’t worn a full slip since I was a child (I hated those things!), but I’ll wear a half-slip depending on the skirt.
The jamming up between the legs thing was weird, though I don’t think I’ve seen any of my skirts do that. I mean, I have some white skirts and dresses, but they’re definitely thick enough that there’s nothing to be seen. I do have one sheerish skirt and I usually try to either go commando or wear underwear that’s lighter with it. Though I haven’t worn it in a while.
I think it might have been windy, and the wind pushed the dress against her body. In the last photo, where she’s hailing a cab, both sides of the dress are touching between her legs, which would not be OK with me on a white dress with nothing underneath. You can definitely see a dark outline in the crotch area, like underwear or pubes. The dress is short and flowy enough that I think going commando could be risky on a windy day. A slip really is the best way to ensure that a dress like that hangs properly and isn’t exposing too much.
I finally actually checked out the shot of the white dress. Maybe it’s my style, but it wouldn’t even occur to me to put something under there. It’d be weirder if there was some sort of slip peeking out by accident than to catch a vague outline of underwear. Meh.
Well, she’s not a Doper, but my 21-year-old sister, who is probably the most modest person I know, hasn’t worn a slip since she was forced to by our mother. If a dress or skirt is too immodest, she has other ways of making them work. Sometimes she’ll even do the Pentecostal cheerleader thing and wear shorts under them.
I have a couple of summer dresses which are white and just a bit too flimsy of fabric to be worn without a slip. I couldn’t find anything suitable (fabric nylon or some such and thus totally inappropriate for hot, humid summer days) so I made a slip myself from a nightie pattern, in 100% cotton. It does need a light iron to get rid of the wrinkles but it’s worth it.
Target. K-Mart. Wal-Mart. Penneys. Sears. Macy’s. Dillards. Kohl’s.
Lingerie department, hanging on hangers right next to the camisoles and the shapers.
Ah, that other thing that I only know as outerwear. 
No slips here.
I wear it with any skirt except long, flowy peasant skirts. Then again, I don’t skirts other than long peasant ones often… or at all, really. (I’m 26.)
Ehhh, we’d probably get inured to the sexy if it was common everyday wear. If everyone got to see them, then they wouldn’t be particularly special, and hence not particularly sexy, either.