Under current law, having actual intercourse with someone between 16 and 18 does not mean that you have to register as a sex offender... But having oral sex with (presumably) somebody between 16 and 18 does mean you have to register as a sex offender
It makes no sense to treat oral sex more harshly than actual intercourse.
It is not proper for the court to decide whether legislative actions are sensible. On the other hand, it would IMHO be appropriate to examine legislative history.
If, e.g., there were a number of sex-related crimes in different sections of the California Code, and the legislature decided to reduce the punishments for some of them, it would be possible that the legislature forgot one. Such things have been known to happen.
Although the state should never be allowed to convict someone for disobeying something that should have been a law but wasn't, it does not seem unreasonable to let someone go or reduce their sentence if there is clear evidence of legislative intent favoring such action.
I have no idea what evidence of intent exists in the extant case.