I never said it was a good thing. Here are some random thoughts. There are manuscripts from around the 10th-12th Century (about the time the Sheelas probably date from) where monks/priests write of the evils of women and that they are dirty creatures that shouldn't even be looked upon etc...
I'm not too sure that this was an attitude change that arrived with Christianity. I think the patriarchal wankness is much older. I think the people who think that there was a more equal and balanced society pre-Christian era are, again, chasing an ideal.
You probably have to go back to the mesolithic, to the hunter gatherer stage, to find any level of equality. I think the introduction of farming and a more settled way of life was when the change of attitude started to occur. That's when the women started to become 'house bound'.
I know there are matriarchal societes recorded, but they are recorded as oddities. People like Boudica and Jean d'Arc are figures that stand out because they are unusual for there times.
As an aside, which speaks slightly against the above to an extent - loads of people come to Tara each year and many women's groups sing the praises of St. Riordain (sp?). He held one of the three synods at Tara. He is often hailed as one of the first women's rights movers - in the 8th century .... yeah right! This is because he got the Irish kings to agree that women should not be allowed to fight in the army. How this is perceived as a move for women's rights is beyond me. Anyway, what it does say is that in Ireland, until the 8th century, women were expected to fight in battle alongside the men. This guy stops them being allowed to and gets praised for it. It's always confused me a little.
As I said ... just some random thoughts.