I think the institution of marriage is brought into some new lights with this play. I am not sure it's attacked. I don't get the idea that the play is saying "Don't get marries." I do think that it is asking us to examine the composition of marriages, the dynamics of it, and to see if we are like Jimmy and Allison- comfortable in our "ruts" and routines, unable to break out and unable to speak freely. I think that the play does show accurately the malaise and numbness that can result in marriage. It seems ironic because marriage is the one realm where emotional sensitivity should be present at all times: One is living with someone one loves and their life is with them. Yet, because of this condition, marriage can become very routine based, as we see in the opening of the play when Jimmy complains of both the predictability of marriage and how Allison would be able to become accustomed to anything. This patterned aspect of living can become a part of marriage, where two people live together, but are actually going through the motions of existence and not engaging in real, substantive emotional connection. If the play is suggesting anything about marriage, it seems to me that it is calling to prevent us to enter this realm of alienation and isolation from one another's emotional compass and rather seek to broaden our compassion, our ability to listen to another person's suffering. Jimmy needed to be an emotional comfort to Allison during her emotional challenges about pregnancy and the miscarriage, although at the end it seems like he shows some sensitivity. Instead of being a "spiritual heathen," perhaps he could have been more insistent on communication that was valid and authentic. In order to do that, though, he would have to overcome his own sense of restlessness and anger that plagues him throughout the play. Perhaps, this is the ultimate message of the play regarding marriage: To be effective with one other person, one has to be effective with one's own sense of self.
I have not read the play Look Back in Anger and I do not know in what way the institution of marriage has been attacked in it. May be there are some valid criticism of institution of marriage as practices today. However, I firmly believe that institution of marriage has been and continues to be a very important and useful institution in life of humans.
Down the ages there have been many changes in the way institution of marrige, and the family to which it gives rise, operate. Such changes will continue and must continue in future also in line with changing realities of the human life including factors such as technology, and economy. However, I do not see any better alternative to marriage.