Dear Bilge Rat —
Knowingly renting your home to a homosexual or unmarried couple is material cooperation in sin, but not necessarily sinful in itself, depending on a lot of factors.
If you cannot find another renter, and you are legally obligated, you would have to rent to them.
I would offer it around to other contacts, friends and relatives.
Of course, a single person could rent your house and then do sinful things in the house that you have no control over. You could also have the house rented out by a rental agency, and that way you are more removed from the material cooperation.
As to your last question, generally, yes, it is not correct. The person may be acting in ignorance.
Consult your pastor about this.