Does the Polish Peasant's Party have any other parties remotely like it in Europe?
There are quite a lot of Agrarian parties in Europe, especially in the East and in Scandinavia.
The big European Socialist/Social Democratic parties date from a time before modern social issues were of any real importance; they were in the '60's (and still are now) mainly parties of what would be called economic issues in the U.S. The New Left was something that, mainly, happend
outside these parties (and outside the two big Communist parties (PCI, PCF) as well) rather than inside them (as was the case in the U.S), at least initially.
Or to put it another way: there's no contradiction between being leftwing and not being socially liberal* because there was no link between these two concepts in the first place.
*A sizeable proportion of the hard-left Campaign Group over here are social conservatives.