I haven't seen the article so some of the points below may be answered.
I would posit a couple of reasons for straight couples being together longer than gay couples.
First, the institutional hurdles to marriage or unions that gay people face become hurdles for straight people to get out of marriages. For example, alimony, community-property divisions and so forth make it more expensive to split. Furthermore, a stay-at-home woman in a straight marriage may have no access to her husband's social security if they split, so there is a cost to splitting. A stay-at-home gay person has no access to his/her partner's social security in the first place.
Second, children. First, there's the stay-together-for-the-children's-sake impetus, which may allow time for straight couples to resolve their issues, but in any case tends to reduce the incidence of divorce. Second, and probably more important, is that children can become the focus of a couple, giving them a shared interest in something for probably a couple of decades of their married lives--something to replace sex, early common interests, etc. It would be interesting to see if gay couples with children resemble more the straight couples with children than they do gay couples without children in relationship longevity.