Megan Fox was appearantly dating now-husband Brian Austin Green while filming Transformers, but that apparently didn't stop co-star Shia LaBeouf. Asked by Details Mag. if he ever hooked up with Fox, LaBeouf nods and said,

"Look, you're on the set for six months, with someone who's rooting to be attracted to you, and you're rooting to be attracted to them. I never understood the separation of work and life in that situation. But the time I spent with Megan was our own thing, and I think you can see the chemistry onscreen."

When asked specifically about Fox's relationship status with Green at the time, LaBeouf replied by saying "I don't know" twelve times, finally ending with, "It was what it was." 

Something tells me that Austin Green knew or suspected something and is okay with it now that they are married.  As a man who is involved with Megan Fox and the drama that comes with Hollywood, you need to carry a deep armor, i suppose.  But, that still is a cheating-shocker!