When two beautiful people are in a movie together, audiences can't help but speculate about the nature of their offscreen relationship. Is that sexual chemistry real? Did they enjoy that staged kiss just a little too much? Actors are professional pretenders, of course, but sometimes that sizzle isn't all fiction. There's no shortage of celebrity affairs, and no shortage of on-set dalliances that ended star marriages.
Usually, celebrities deploy evasion tactics when dealing with rumors surrounding their on-set affairs, especially if they're in new relationships with their costars. Stars sometimes won't get personal, even if it's obvious that it's their movies that led to their real-life divorces. (See: Brangelina perpetually giving the press murky details about their interactions on the set of Mr. & Mrs. Smith.) Celebrity divorces are messy, and destroyed marriages are difficult. There's no reason to give lawyers more material to work with.
Old Hollywood stars were less coy about their on-set dalliances, and would even sometimes play up their affairs in front of crew members to drum up press for their movies. It just goes to show you that celebrities who cheated and got divorced are nothing new.