It's highly recommended to only take antibiotics when needed and when prescribed by a doctor.
Taking antibiotics encourages harmful bacteria that live inside you to become resistant. That means that antibiotics may not work when you really need them. Because antibiotics alter the bacterial makeup in our bodies, they can leave you more vulnerable to yeast infections and other types of fungal growth. Some research has shown antibiotics can lead to an increased chance of catching viruses. Also, taking antibiotics when you don't need to increases your chance of having side effects from the drug such as: Nausea, Diarrhoea, Dizziness, Yeast infections, Vomiting etc.
Taking antibiotics pre-exposure definitely won't be enough to prevent you from catching chlamydia and isn't recommended.