It's also likely to be wrong. Most STI transmission involves asymptomatic spread or very delayed on set of symptoms. The causation fallacy is very much in play here. In other words, the likelihood that the source of an infection was the last person you saw is not all that high. So, unless you have done genomic testing, claiming you acquired an infection from the last person you saw is more likely to be wrong than right. The epidemilogical variables around transmission are considerable and there is much uncertainty. It's also an area where data is patchy and individual variation is high. If you want to find out more, google the Kirby Inst in Melb as a good source of reliable information.