In 2009 Megan Fox did publicly mention this in an interview with Esquire magazine, and provoked plenty of comment about it in the media. She mentioned having dated a woman in her teenage years - a stripper, according to her account.
However, she has been in a relationship with actor Brian Austin Green for 8 years now. They married in 2010 and now have a baby son. Fox's name has never been linked with any other woman and there have never been any more details about her teenage affair. So it may have been a one-off, or she simply didn't meet anyone else she fancied, or just has preferred to be in the relationship she is in. Some people even think she invented the story to get publicity, but there is no reason why it shouldn't be true - quite a lot of people are attracted to both sexes, either throughout their lives or just occasionally.