A person who opposes lacrosse or works against the sport's promotion.
From antilacrosse + -er (one who does something), following the pattern of adding -er to create an agent noun.
While this is a constructed term, real antilacrossers existed—school administrators in non-lacrosse regions who resisted the sport's expansion, or community leaders opposing team culture.
Complete word intelligence in one call. Free tier — 50 lookups/day.