How widespread is the strategy of, "Let's get tested together before we have sex, for a variety of STDs"?
Part of standard safer-sex counseling has been to advise individuals to either use condoms every time they have intercourse or to first rule out the presence of any HIV or other STI and then make a commitment to monogamy before foregoing condom use. This means that two people would have to test for HIV/STIs more than once over a period of six months (or more). If both people are HIV-negative and also free of other STIs, then that couple may choose to have unprotected sex with each other, provided they have been monogamous with each other over the period of testing and are committed to remaining monogamous.
To our knowledge, there are no data on how many people follow this strategy, which has sometimes been called "negotiated safety." However, overall numbers are probably not of as much concern as the specific situation of any particular couple. It can be challenging for couples to accomplish this and remain faithful to their monogamy commitment for the long term. Part of the agreement some couples make is to commit to communicate with each other, if and when the monogamy commitment is broken. Such communication can be difficult, however, particularly if it comes at a time that the relationship is also under a strain.
So, the bottom line is that one cannot get HIV or any STI from someone who doesn't have it. If two members of a couple are disease-free and remain so, then unprotected sex should be risk-free in terms of transmission of STIs. However, as mentioned above, this will only remain true if both people remain free of STIs and that requires ongoing trust that there are no outside sex partners or that all sex with other partners is 100% protected.