Although it has yet to be confirmed on the English site, multiple sources are stating that the same-gender marriage mechanic that appeared in Story of Seasons: Friends of Mineral Town, known as a "Best Friends Ceremony" (uh huh) will be appearing again in Story of Seasons: Pioneers of Olive Town, as confirmed on the Japanese Story of Seasons website.
Ranch Story Blog, the Ranch Story Wiki, and ScreenRant all confirm the return of the much-requested ability to marry anyone regardless of gender. Over on Reddit, user jpdt_ligiiihh translated the screenshot above, taken from the Japanese site:
"There are 5 bachelors and bachelorettes to romance, making them 10 romance options in total. It is possible to become partners with the same-sex through a 'best friend ceremony'."
Story of Seasons: Pioneers of Olive Town will launch on Switch on the 23rd March. Pre-orders will receive a squishy buffalo plush.