“P.S. I’m a gaijin” no shit…
In answer to your question, it’s fine to go alone as long as you use your common sense and stay safe. If you’re planning to hook up, go to a love hotel rather than the guy’s house. Keep your items safe, etc. (there are usually lockers in clubs) and make sure you know how to get back home/to your hotel.
I don’t think anyone actually notices/cares if you go alone. It’s difficult to hear your friends anyway once you’re inside.