Its fine, but run it on petrol every now and then. Petrol will lubricate the valve seats and prevent them wearing down. The USDM Civic NG comes factory fitted with hardened valve seats, which are resistant to wear and tear. Petrol engines don't have them.
Just for comparison, my Cultus has been run most of its life on CNG. Now at 310k km, its wet compression is 180/170/120 psi (it was measured back at 290k km). It also burns oil. On the other hand, my dad's Civic 2004 has been run mostly on GT road and motorway on petrol, with CNG mostly used in city. At 250k km, it runs fine, almost as good as it did when bought at 100k km.