Nope, you cannot have sex even if you use a condom. I asked the doctor the same question and he said that you can have sex but to make sure that the chlamydia is completely gone he strongly advised not to have sex until the treatment is over. It makes sense, I mean if your gf accidently gave it to you even if you guys used a condom, you would give it back to her and the treatment would have been pointless and you guys would both have the disease thinking she was cured. Hope this helps !