Men moan about women being easy when they can't get laid. That's when they notice how easy it is for other guys but not for them ;)
They get resentful and jealous. Instead of complaining about all the easy guys (you can't have easy girls without easy guys, you know) they complain about easy gals.
So you are saying that men complain about a girl being easy out of frustration.
Good try, but wrong answer...
For clarity's sake)) ... I'd say men label a girl as easy when:
1) gives out without putting up any defences
2) gets drunk and gives out
3) goes straight to bed with a random guy of no quality
4) gives out despite there being no commonalities or attraction to the guy
5) sleeps with a guy of a lower status
6) gives out despite having a boyfriend
7) is going out with more (2, 3, 4 etc.) guys at the same time
8) in record time!!!
9) ... feel free to add :-)
This is only a provisional list. But I'd say Polish girls, or rather the reputation they have (trying to be politically correct here), would tick all of these boxes..