It’s an allegory for the civil rights movement with prof. X as MLK Jr and Magneto as Malcolm X. In proper allegorical fashion this means it can be representative for any form of civil resistance by a repressed class, including the fight for gay rights.
50% of what Thanos considers life since it was powered by his will. Since he seemed to imply that nature (plants and animals) where not part of this it’s safe to assume it was sapient life only.