Sub-group analysis of data from the project “Time stability in reducing the risk of HIV transmission among homosexual men in Germany” (Zeitstabilität in der Risikominderung der HIV-Übertragung unter homosexuellen Männern in Deutschland, 8th follow-up survey Gay men and AIDS [GMA] 2007) on the topics sexual behaviour (number of partners, condom use), drug use, taking special account of different age groups, birth cohorts, social strata, HIV-testing behaviour, sexually transmitted deseases