I've been thinking more about mixed-orientation marriages and I'd like to add something that I think plays a big part in why people opt for getting married...
Especially within the LDS Church, family is of the upmost importance. That's the whole goal. Grow up, find an eternal companion, get married in the temple, have an eternal family.
Having no desire to have children of my own, this is something I don't experience personally. But I can see how that would play a big role in entering into a mixed-orientation marriage.
I'm still processing and may have more to say later, but this is all I've got for now.