'The lifetime average number of sexual partners for Poles is 4.28' That's it??
It seems a bit low, but not too low.
4-5 partners may be quite common having in mind that plenty of Polish people marry young, countryside is rather conservative (and gossips spread quickly) and people generally tend to be involved in long-term (>5 years) relationships.
The "average" however doesn't seem to be right. There are still people who are much more active - and as much as 100 may be too much but two-three dozens is quite likely, especially if someone spent a good few years "studying" in a big city.
Also, Warsaw from what I know is quite promiscuous which should increase the average for the rest of the country a bit :D