It's actually a very wrong idea and a proof that human uneducation can lead to serious catastrophes. The idea might seem nice, but it is harmful in practice. Like communism.
Authors of the site probably are not aware, that even if two people have the same std, they might suffer from so called superinfection, which is even more harmful than the standard, normal infection. Besides there are lots of people who are actually turned on by the thought of being infected with a deadly virus, such as HIV. And they would probably do it if it was easy to do and they were pretty horny, but they would regret it till the rest of their lives. With such portal, they can really easily find a match and do a mistake they will actually regret, a mistake that will ruin their lives.
If someone is sick, he/she should get some help. If there's no help, then that person should at least inform every close person to him/her that he/she's sick. Sites like that are probably going to end up in some local std pandemics.