The numbers went up because people started coming out of the closet and admitting that, yeah, they were doing something that was previously illegal but was now legal.

It doesn't mean that fewer of them were doing it before it became legal. They were doing it and hiding it.

It's the same as when gay people finally started coming out of the closet, once the laws and stigmas against being gay were reduced. Now they could finally admit what they were doing all along, with less fear of being stigmatized, arrested, imprisoned, or killed.