Complex sentence: Rosenberg also outspokenly opposed homosexuality--notably in his pamphlet "Der Sumpf" ("The Swamp", 1927)--he viewed homosexuality (particularly lesbianism) as a hindrance to the expansion of the Nordic population.
Split into simpler sentences: Rosenberg also outspokenly opposed homosexuality--notably in his pamphlet "Der Sumpf" ("The Swamp", 1927). He viewed homosexuality as a hindrance to the expansion of the Nordic population.