C.J.
While I would love it if my sons abstained until marriage for these exact reasons, I am no dummy to think they will. If you are sexually active then yes it is important you are tested for a variety of diseases. I also feel that if you are tested for HIV it brings home just how risky unsafe and multiple partner sex is and could result in teens really thinking twice about their actions not because mom and dad said so, but because they are really aware of their health.
I know they are teens, but some may say that about kids and handwashing. I'll tell you what, my kids are very aware of how important washing their hands are in keeping them healthy. . . I think it is an extension of that very lesson that could be applied here.