Interesting rumour. I'm slightly interested to know what your basis for this assertion is
Half a dozen of my friends figured this out the hard way. Their doctors all traced it back to drug interactions between antibiotics in the -cycline family (minocycline, tetracycline, etc.) and their low-dose oral contraceptive pills. But since anecdotes aren't proof, I spent about five minutes Googling:
Reference 3 (about 3/4 down the page)
Many sources I found note that it is difficult to conduct formal research in this area because women don't want to take antibiotics as part of a study and risk getting pregnant. It is difficult to prove what happens, but my friends have traced it back and told me what their doctors said. I hope sexually active readers hear this and protect themselves.