To be honest, I don't like all the suggestive qualities producers are starting to flood their films with. Movies used to be something we could go watch with enjoyment in mind, but of late I'd need to check on the content within it. This is emphasized by cinematic creations such as Transformers and X-Men: First Class; while both were excellent films, they certainly held a lot of surprises which I didn't certify. But in the end it hasn't affected me any, because I choose it not to...that, or I've seen enough of it over my life that I simply don't care.
I guess that depends a lot on the mentality that the producers had. The two examples you mentioned were both big budget Hollywood productions, so they are clearly going to look for ways to attract an audience. I haven't seen X-Men: First Class, but from Transformers alone, I can tell you they had a really defined idea of who they would attract with that. The shot at the start of Transformers 3 was clearly just for people who want to see a young woman in underwear. It didn't really have a purpose.
But there's different cases. Others use things like that from angles that actually enhance the sense of realism in the film. But that's mainly art-house/indie filmmakers or maybe 1 or 2 that somehow got Hollywood to back them up. If it's the kind of movie that was clearly meant to just get money, then you bet they will use things like that to lure in their intended audience, you can usually tell by the rating and genre. (For example, if it's an acton movie and it's PG-13, you're likely to see women in provocative clothing, but that's mainly in most cases I come across)
when I was a kid it wasn't uncommon for there to be nudity in PG-13 movies...
It's actually allowed under certain circumstances. Have you ever heard of Titanic? That one had a full-frontal nude scene in it, but it got a PG-13 because the scene was "artistic".
I've haven't heard that case many times since, but I guess it has a base. It's more based around tastefulness and context, or at least that was how they justified the scene. Showing a woman naked in a more sexual context, though? Now that has always earned R ratings as far as I know.
EDIT: This might help.
MPAA rating info said:
More than brief nudity will require at least a PG-13 rating, but such nudity in a PG-13 rated motion picture generally will not be sexually oriented.
Here's the rating info, just in case anybody has questions.
http://www.mpaa.org/ratings/what-each-rating-means
The F word is used in PG-13 movies just once. Too much would be R rated.
I've heard that it depends more on the way the word is used. The F word has many uses, you know. It's actually known as one of the most versatile words in any language.
It also has a sexual use, as you may know already. That is usually the one I've seen they frown upon. Most other uses of it are supposed to be fine as long as it's not overdone. But I've seen some PG-13 movies get away with 3 or more uses of the word in it, maybe that's because they weren't in the sexual sense.