First off, let me say this is the musings of (hopefully) a future theologian, not a real scenario!
Would it be morally licit to obtain a civil union (either heterosexual or homosexual, and feel free to make distinctions between the two if you think it matters morally) with a friend entirely for tax benefits? Consider it in a case where the benefits make the difference between barely surviving and having to cut back on essentials (food, etc.).
Secondly, would it be morally licit to adopt a foster child with a member of the opposite sex who is just a friend, if both members are not called to marriage, in order for the foster child to have a home, despite the fact that there would be no marriage? I use foster child since adoptions already have backloaded Interest from married couples, whereas the foster system does not have much interest from families sadly.
Thanks and God bless!