This one's not so bad. I get it. No hookers. Don't bring hookers inside this place. You couldn't really disobey this sign and then say you didn't get the gist of it without looking like an ass.
This sign is probably on the outside of a Japanese love hotel. And, in this dodgy area, I'm sure the foreign hookers wait on the road to get business from drunken guys leaving bars and izakayas. Although Japan is generally a very safe, community-like place, I would be sure to steer clear of the dodgier parts of the city at night. These can be recognized by the abundance of flashy lights, crowds of young drunken people, countless bars, izakaya, and liquor stores, and the general cheap and/or suspect looking massage parlors and love hotels.
It may be interesting to note that the Japanese are usually pressed for alone time. Many young couples looking to get intimate utilize love hotels as if they were a vending machine. In fact, many of the love hotel electronic room choice boards resemble that of a vending machine.