Complex sentence: FTM International (FTMI) is the first, largest and oldest exclusively FTM (female-to-male) organization in the world and is largely responsible for the modern acknowledgment that sexual orientation and gender identity are different things.
Split into simpler sentences: FTM International (FTMI) is the first, largest and oldest exclusively FTM (female-to-male) organization in the world. They have worked to promote acceptance of the idea that sexual orientation and gender identity are different things.