Serosorting is listed by the CDC as a viable tool for protecting oneself against HIV infection. While the CDC touches on the inherent risk involved in knowing with 100% accuracy the status of oneself and one's sex partner(s), this point cannot be stressed enough. In order for there to be zero chance of HIV transmission to an uninfected partner, there must be 100% honesty between sex partners about their HIV status, the last time they were tested, and every sexual encounter they may have had since that last test was taken. If there is ANY chance that one partner might have been exposed to HIV by any means since their last negative HIV screening, then serosorting alone is NOT assured to prevent transmission.
Putting aside the unreliability of serosorting as a prevention method, it has a much wider impact on the gay community at large. Serosorting has been referred to as serodiscrimination or serosegregation by many people living with HIV. It has been observed to promote fear and stigma among the HIV negative members of the community who believe that simply avoiding sexual contact with openly HIV positive partners will protect them from becoming infected. And while that may be partially true, it does not take into consideration the fact that many people who report their status as HIV negative either do not know their recent HIV status, have not been tested recently or at all, or are simply lying about their status in order to appear more attractive to those afraid of having sex with HIV positive partners.
It is through these types of encounters found across dating sites and hookup apps like Grindr, Adam4Adam, Scruff, Growlr, GuySpy, BarebackRT, Tindr, Hornet, etc. that HIV stigma can lead to greater numbers of new infections among gay and bisexual men because it promotes fear of rejection, fear of infection, and destroys most attempts at having open, honest disclosures about status. Men who are living with HIV and taking antiretroviral therapy to maintain an undetectable viral load are as safe to have sex with as one can find. Instead of shaming them back into the closet by promoting serosorting, thank them for their honesty and consider the fact that they are protecting themselves and others by adhering to their therapy.
Putting the bias and discriminatory elements involved with serosorting aside, the practice carries with it heightened risk of other sexually transmitted infections due to a false sense of security from HIV. Men who have sex with men are reporting an increased rate of infection with gonorrhea, syphilis, and chlamydia after having unprotected sex, most often while serosorting. Bacterial superinfections are becoming more and more prevalent within certain networks of sexual partnering, especially those where recreational drug use is a factor. Talk to your partner about STIs and make informed decisions, not based on fear or shame, but on facts and available options for prevention.
See also:
http://sti.bmj.com/content/93/1/71
http://sti.bmj.com/content/89/Suppl_1/A188.4
http://edoc.rki.de/oa/articles/regx7ntJuQKME/PDF/28zHkrmw5XlkU.pdf
https://wwwn.cdc.gov/hivrisk/decreased_risk/communication/same_status.html