Antivirals can have pretty nasty side effects. I haven't managed to refine my search terms on PubMed adequately to get you some examples yet, however. It would not be something that an otherwise healthy person would want to take, when the alternative methods for reducing risk are much safer. As TVP45 suggests, condoms, and other barrier methods, are highly effective for preventing STD transmission. However, a much more effective method is simply to require STD screening in all your partners before engaging in any form of sexual contact (and if you have had prior partners for whom you do not know their sexual history, then you too should be screened). Only engage in sexual contact if they test clean. (And of course the only 100% safe method of avoiding STDs is to avoid sexual contact entirely, but not many people choose to follow that advice.)