Two differing elements in one contractual relationship, basically. Some easy examples would be, racially different, religiously different, or even a differing set of sexualities. Sometimes, the first is included, very rarely the second, and almost always the third. I assume, you, mean the latter. In that case, to give a basic example, a male and a female, both homosexual, agree to marry, for whatever reason -- tax benefits, convenience, 'a cover up', seeming the most popular motives.
If I got married, mine would be. My gf's straight and I'm... less than straight. But I do still like girls so it could work.
Differences in sexuality. Let's say that Bob is bisexual and he is married to Pam, who is Pansexual. Their marriage would be of mixed orientations.
Ohh So if like Kimmy is a lesbian and Cherry is bisexual and there married that's a mixed orientation marriage too