Why were there (apparently) more homosexual men in Ancient Greece than today?

In traditional Modern Greek society, there was a stigmatised group of gay men: poustides.

In traditional Ancient Greek society, there was a stigmatised group of gay men: kinaidoi.

The stigma in antiquity was more about being a bottom (i.e. about power) than about having sex with males per se.  What was not stigmatised was homosexual activity with boys (which did not necessarily include full-on intercourse), as part of a mentor-like arrangement.

The stigma in Ottoman Greece was certainly etymologically about being a bottom: πούστης < Turkish puşt was a bottom, κωλομπαράς “arse-fancier” (Greek–Persian blend) was a top. Quite possible that in traditional Greek society both roles were stigmatised, haven’t studied it in depth.

But there were homosexual men in Ancient and in Modern Greece, and I don’t see a particular reason to think there were more gays in antiquity. A particular, quite regimented set of behaviours, which was as much about mentoring and social alliances as it was about sex, was licensed. That didn’t mean Ancient Athens was Big Gay Al’s Big Gay Animal Sanctuary. 

To my amusement: Greeks in the 60s were convinced London was Big Gay Al’s Big Gay Animal Sanctuary. Because Britain moved early to decriminalise gay sex.