One of the most critical parts of polyamory is being completely open, honest and truthful about sexual history and activity. A polycule is only as stong as the weakest link, it just takes one person doing something irresponsible to bring the whole world crashing down.
However, there is another side to this argument around the subject of control, I read the following blog post a while back and recommend you take a look.
The usage of fear of STI’s to control another persons behaviour is not ethical, being able to make an informed decision about your own expose is a completely different matter.
Its worth noting that at the point you kiss someone, you have effectively become fluid bonded without necessarily realising it.
“One of the most common questions polyamorous people hear is: but what about STIs? It is a valid question, but when most of what people think about STIs it is hard to sort out fact from fear.”