I guess there's no more reason for straight couples to get married than gay couples, but marriage has always been a more heterosexual thing. Many of the gay guys I meet are of the opinion that marriage is something to be left to straight people.
That's a bit narrow-minded of them.
Aside from child rearing I can't see any reason for marriage that isn't sentimental and a bit silly to be honest. No one stays together because a divorce would be too expensive.
The simple fact that you have more rights and protection regarding your spouse goes a long way. Imagine arriving at a hospital and being told that you can't see your loved one because you are not considered "family."
Immigration is another aspect--governments are a lot more inclined to respect the seriousness of a relationship by way of marriage than they are one of convenience.
Insurance/financial/disability/medical/whatever benefits.
Adoption, legal status with any step-children, being able to make medical decision on behalf of your spouse, inheritance, etc.
Marriage is hardly just a "silly thing."
As I said in my earlier post--marriage affords many rights, responsibilities, etc and offers protection and respect in the eyes of the government/law/whatever.