In short, back then Law and Justice was a ruling party and they are very homophobic (see https://en.wikipedia.org/wiki/LGBT_rights_in_Poland#Law_and_Justice htt

In short, back then Law and Justice was a ruling party and they are very homophobic (see https://en.wikipedia.org/wiki/LGBT_rights_in_Poland#Law_and_Justice https://en.wikipedia.org/wiki/LGBT_rights_in_Poland#Law_and_...). They were worried Charter of Fundamental Rights might force Poland to grant homosexual couples the same kind of benefits which heterosexual couples enjoy. Opting in after Law and Justice opted out is pretty much impossible because two-thirds majority is needed for that, and Law and Justice would block attempts to do so.