Ex post facto laws are forbidden by the US Constitution, both at federal and state levels. [1]
At least, at this time, with current interpretation- "the Supreme Court has explained that people must have notice of the possible criminal penalties for their actions at the time they act" [2] (See also Weaver v Graham[3])
The laws are not ex post facto. They have been on the books, on some cases for over 100 years. States were not enforcing them, because they believed they were constitutionally prohibited from doing so.
At least, at this time, with current interpretation- "the Supreme Court has explained that people must have notice of the possible criminal penalties for their actions at the time they act" [2] (See also Weaver v Graham[3])
This might be subject to change.
[1]https://constitution.congress.gov/browse/essay/artI-S9-C3-3-...
[2]https://constitution.congress.gov/browse/essay/artI-S9-C3-3-...
[3]http://cdn.loc.gov/service/ll/usrep/usrep450/usrep450024/usr...