In this day and age, a lot of unmarried couples live together. While this is perfectly acceptable and should never be a form of discrimination based on your religious beliefs, landlords need to be careful to treat both adults equally and fairly when it comes to the tenant screening process.
Assuming both are adults over age 18, always have both parties fill out a rental application form, and run a credit check and background check on both of them. You never know what will turn up. Depending on how long they’ve been together, there’s even a chance one person’s past isn’t known by the other person. That’s not to say it’s up to you to disclose anything negative you find, although they are entitled to know why they are being denied if you do decide to reject their application based on something that turned up during tenant screening.
Also, get both parties to sign the lease. Even if one says he/she will be carrying the burden of the monthly rental payment, there’s no guarantee they will stay together. If one moves out, the other needs to be on the lease and therefore legally bound to uphold the tenets of the lease agreement.
No matter what they call each other, to you two unmarried tenants are merely roommates, and should be treated accordingly