I think you will find the West takes a different view of marriage in general.  We often claim that we marry because we want to show our love for our partner or because we want to recognize the union of two people.  There are many couples who marry and choose not to have children.  In general, procreation is not seen as the primary purpose for marriage by society at large.  There is great debate right now about same sex marriage.  I do not think you would find the same debate about the marriage of middle sex or eunuch peoples.  Perhaps this is because it is less of a public issue.  Most people with these conditions keep them quiet.  A same sex marriage is a little more obvious to the public eye.  Again, many societies do not see marriage as a vehicle for procreation so they would not take issue with a couple marring that would not produce children.

Islam is a religion, and as such, its ideas regarding marriage and its purpose should certainly be upheld by those who practice it.  However, there are two societal interests in a marriage, those of religion and those of the state.  In any country that is not a theocracy, the principles of any particular religion should take no part in the regulation of marriage. 

I believe like other "mixed" marriages, same-sex marriage is here to stay in the USA. As the previous post mentioned, married couples are not required to procreate, so it would be ludicrous to use this excuse exclusively for same-sex unions. I have more of a problem with the financial and tax benefits that such marriages extend to its couples, but I believe that the personal freedom aspect overrides such considerations.

Of course, we in the West have a great deal of conflict over this issue so there will be many different opinions about it.  My own view is that we do not limit marriage to the procreative function.  We do not force people to get divorced if they cannot procreate.  We do not require them to promise to have children in order to marry.  Therefore, we should not disqualify homosexuals from marrying simply because they are not going to procreate.