In many cultures it is not acceptable for unrelated, unmarried adults of the opposite sex to share accommodations.

Sometimes this is a way of hiding group homes for otherwise undesirable tenants (sex offenders, recovering addicts, etc).

As long as it's not in the middle of nowhere or next to a registered sex offender, you'll get plenty of viable offers.

In the book "Ignore Everybody: and 39 Other Keys to Creativity" the author discusses the concept of "Sex and Money".

People get to wear shoes in their homes, walk to their cars without being harassed, and even occassionally get to have sex, without having to live in fear.

In my experience, I found out that they had a criminal background, previous evictions, sex offender records, and generally they try to do a power play to establish control over the landlord/rent collector.

http://www.naplesnews.com/news/2014/may/21/woman-arrested-after-sex-rent-deal-goes-bad/She was supposed to be faithful to him when he was in town.

I require adequate income proven by paystubs, plus I check the sex offender registry and run a criminal background check. 

You may be able to check if they appear in any public registries such as a list of sex offenders. 

Unfortunately, not all the illegals are good hearted people with good intentions and many are cartels that traffic drugs and sex, gangs, spies, terrorists, etc.