Why do we call it yayin nesech”
Yayin nesech literally means wine that is poured (by non-Jews onto the altars of their particular avoda zara). As there is little in the way of true A”Z these days (though there are arguments that can be made for the existence of A”Z in many cultures), the wine touched by non-Jews is referred to as Stam Yeinam (simply their wine), and it i still assur to drink, even if never involved in A”Z ritual, because drinking wine with non-Jews leads to socialization with them in such a way as can lead to possible intermarriage.