Common sense dictates that the best way to avoid an embarrassing mammary incident is to keep one's nipular region covered while out in public. But Sienna Miller apparently isn't concerned with being sensible.

The London Guardian reports the starlet is taking legal action against two British tabloids and a photo agency for publishing pictures of her frolicking topless -- a lot -- with still-married squeeze Balthazar Getty.

The starlet's legal pit bulls are suing in London's High Court for breach of privacy over the many intimate shots snapped during her getaway to Italy's Amalfi coast with the actor-cum-oil scion, whose pectoral tattoo of his wife Rosetta's name is visible in many of the photos.

Among the excruciatingly personal pics: Getty is captured giving Miller what looks to be a very thorough breast exam, and she's seen seemingly popping a zit on his face.

Of course, Sienna wouldn't have needed to get her lawyers involved if she'd just, you know, worn a shirt. Or if she and Balthazar had found somewhere a tad more private to hold their grope-fest, like, say, a room with four walls and some curtains on the windows (also keep in mind that they continued to cavort even after the first round of photos went public).

And it's not as if Miller is naïve when it comes to the always-hovering paparazzi. In December, she won an estimated $75,000 payout from two British tabloids after they ran "intrusive" photos of her shooting a nude scene al fresco for the upcoming film "Hippie Hippie Shake."

This week, Getty decided to break his silence about the state of his eight-year union "in light of the fact that many pictures have surfaced in print and on the Internet which has caused myself and my family great embarrassment," confirming that he and Rosetta, who have four children together, including a 9-month-old daughter, have separated.

A mole reveals to People that Rosetta was "blindsided" by Balthazar's involvement with Sienna, which is rumored to have begun a couple of months back.

Last week, an insider told the mag that the Gettys had been "living separate lives for a while, so Sienna isn't a home-wrecker," but a confidant is now taking issue with that assertion.

"Balthazar and Rosetta were in no way separated when Balthazar began his relationship with Sienna Miller, and any implication otherwise is an absolute lie," rails a "close friend" of the couple. "Balthazar's affair has been utterly devastating to Rosetta and her family."

Sources say Mrs. Getty learned about the romance just before their boobie-packed interlude.

"She is horrified," a Rosetta snitch pipes in to Us. "She doesn't know who he is anymore."