I think that marriage is shown to be complex. Certainly, marriage is shown to house some fairly emotionally brutal moments. Jimmy's tirades on Alison and the cruelty he displays towards her would indicate that marriage is the home for brutality. Yet, I don't think that Osborne is attacking marriage in the drama.
I sense that there is a redemptive quality to marriage by the drama's end. Helena's usurping of Alison has been repudiated. Jimmy has understood that, according to his standards, Alison has finally felt something emotional and they do reconcile. They are drawn back to one another at the end of the drama. Finally, marriage is shown to be where reconciliation in terms of Jimmy's hatred towards himself, the world, and those in it can be possible. While Jimmy might do nothing but spit venom at Alison, his retreat back to his game of bear and squirrel with Alison helps to enhance how marriage is a source of redemption for his own sense of identity. Consider when he looks at Alison and tells her, "I may be a lost cause, but I thought if you loved me, it needn't matter." It might be a moment where marriage is shown to hold some level of potential in redeeming those who might be perceived as being beyond redemption. Given this, I think that marrage is not attacked in Osborne's drama, but reclaimed by its end.