It's because with proposed section 156 the intention is to preserve the power to prosecute historical conduct that we still believe is reprehensible. This includes same-sex abuse of children by priests in orphanages, and that kind of stuff. There's no attempt to make that out of the criminal law's reach when we repeal 159.
The problem with 156 is that it relies on today's age of consent of 16 as the basis for deciding whether something can be expunged or not. To me, it's unfair that in the past, when the age of consent for “ordinary” different-sex intercourse was 14, it feels problematic to me that historically we're in a sense lowering the age of consent for anal intercourse, but only to 16. There is actually a group of people who had sex—an 18- or 19-year-old having sex with a 14-year-old—and if they had been a boy and a girl, that would have been perfectly legal.
If it were two men, and if 156 passes as it's currently drafted, there's the potential that you could prosecute for sodomy or anal intercourse, because under today's law a 14-year-old cannot consent to someone more than a couple of years older.