Gremmi: Unless your doctor's specifically told you, it's a myth that you can't drink on antibiotics. There's only certain types that it's not recommended to (specifically ones that go through the liver). So I'd just look up the type you're on and seek the advice of a medical professional, if you haven't already.
It's not really a matter of whether or not a drug passes through the liver- for any oral medication the liver will be the very first organ the drug sees (blood from the gastrointestinal tract goes through the portal vein, which feeds directly into the liver before going to the rest of the body). It's more a matter of whether the drug or any of its metabolites are toxic to the liver to a meaningful degree, or whether rapid metabolism in the liver is needed to prevent the buildup of toxic metabolites (a good example of this is with acetaminophen). Alcohol puts a fair amount of oxidative stress on the liver, so it's not able to metabolize drugs as quickly (which causes problems if a certain rate of metabolism is necessary to keep the drug or its metabolites from building up to toxic levels).
Commonly used antibiotics, such as many beta-lactams, are very safe with almost no associated tox, so drinking alcohol while taking them isn't much of a concern. However, for a while small amounts of acetaminophen were often included in the formulations of many drugs (including some antibiotics), so drinking while taking these formulations actually would be a problem (due to the acetaminophen); this practice has been in decline, though, so isn't as much of a concern anymore. It's still a good idea to avoid drinking while on anti-biotics, though, for the reason that orcishgamer mentioned- it's just not a good idea to put your body under additional stress while it's trying to recover from an illness.