What made this photo completely illicit in 1953?

This:

kiss 1953 gay

The picture is from the ONE National Gay & Lesbian Archives. Their site explains that it is:

The oldest active Lesbian, Gay, Bisexual, Transgender, Queer, Questioning (LGBTQ) organization in the United States and the largest repository of LGBTQ materials in the world. Founded in 1952, ONE Archives currently houses over two million archival items including periodicals, books, film, video and audio recordings, photographs, artworks, organizational records and personal papers.

Check out the rest of their very interesting archives here.