But aren't these statistical models based on generalisations about what men who have sex with men actually do?
To some extent, yes, they are. However, epidemiologists and statisticians have to make some generalisations about sexual behaviour as they are unable to look at risk-taking on an individual level when assessing risk across populations.
Epidemiology relies on statistical projection to attempt to predict the course of an epidemic and this relies on theoretical principals. However, we believe that the assumptions made in the most recent review were based on the best available research into behaviour and high risk activity at the time.
It is unfortunate that generalisations have to be made and that people have to be categorised and grouped, but we accept that in this instance it is not done in a judgemental or discriminatory fashion, but is necessary in order for sensible decisions about safeguarding the blood supply to be made.