It's perfectly fine in Japan.
After all, this is a country where people participate in events like goukon, omiyai, konkatsu, etc.
The bigger sites like :
So-net 恋人探し | So-net
are only available for people who live in Japan.
They also screen their applicants by asking them to provide a 独身証明 (proof of single status), salary letter, copy of university diploma, etc.