This is a leftover from when sex between men was illegal. The age of consent of 18 is two years higher than the age for all other people. It is not tied to when a person is capable of consenting. By setting the general age of consent at 16, the law already recognises that men and women can consent to sex at 16. A gay man can be convicted even if he genuinely thought his partner was over 18 and this was a reasonable mistake.
Reasonable mistake is a defence for other people.
There are 2 complete sets of provisions in the criminal law in NSW dealing with the sexual activity of young people. The first set is gender neutral and criminalises sexual activity with young people under 16. They apply to sexual activity with both boys and girls and protect each equally. The second apply only to sexual activity between males. Current laws criminalise the sexual activities of 16 and 17 year old gay men.