Heh, I like the ancestral sterilising man-flu hypothesis!
There were actually two Great Y Haplogroup Replacements in Western Europe, and they were at their most dramatic in Britain.
12,000 ya: Western Hunter Gatherers (Cheddar Man)
6,000 ya: Anatolian Farmers (Monument Builders)
4,500 ya: Steppe Herders (Beaker People)
An outstanding feature of both incoming populations is that they had strong social hierarchies, showing significant differences in diet and burial style between the elites and the lower orders. The herders brought intricate metal ornamentations, reminiscent of their Yamnaya origins.
I'm going off the idea that farming is to blame for patriarchy, partly because the beaker people were less attached to farming than their neolithic predecessors. They were animal breeders by preference, coming from a nomadic background. It was probably more to do with overt displays of wealth and power. The harem theory seems likely - or a combination of that with widespread rape, even more likely imho.
I've just got an economic history book called 5,000 Years of Debt. The author thinks patriarchy came in with a transition from 'human economy' to 'money economy'. I'll be interested to see exactly what he means.