The reasons are exactly the same for gay couples as they are for straight couples: commitment, love, stability, security, children, etc.
There is no such thing as illegally married gay couples. There are only legally married gay couples (with or without state recognition), and unmarried gay couples.
married gay couples are prone to the exact same illnesses that married straight couples get.
Many same-sex couples want to have the benefits and protections associated with legal marriage. Many gay men do not want to marry.
It is not legal for gay couples to get married in Greene County, Virginia.
It is not legal for gay couples to get married in Greensville County, Virginia.
It is illegal for gay couples to get married in Hanover County, Virginia.
They don't want to for the same reason that a straight man does not want to marry a straight man.
Of course, employers should treat gay couples and heterosexual couples the same way. If it's to the company's advantage to offer partner benefits to unmarried couples, it should offer those benefits to both heterosexual and gay couples. In many jurisdictions, companies may be required to offer benefits to spouses of employees. If this is the case, they should offer the same benefits to married gay couples that they offer to married straight couples.
This differs from area to area. In some areas, gay marriage has all the same rights and benefits as normal marriage. In some areas, gay marriage is not recognized, and so gay married couples have no additional rights or benefits.
As of August 2013, gay couples who are legally married (anywhere) will be recognized as married by the IRS and the Federal government.
Yes if a gay couple is married, they can get a divorce. Gay married couples started getting divorced about 7 months after gay marriage was legalized in Massachusetts. There is no accurate divorce rate in the united states for gay married couples. Note same sex-couples can generally seek a divorce within jurisdictions that have redefined marriage to include same sex couples. In the vast majority of the world, same sex unions have no legal status as marriage, thus, in general, the courts cannot dissolve a union that has no legal existence within their jurisdiction.
Gay marriages are the same as couples of the opposite sex getting married. The gay couple may make a few changes of their own when they take their vows as do straight couples.