Is it true that when we eat pork, it affects our bodies in complex ways vs grass fed beef, chicken, fish? Do we "need" meat?

No, but... Nothing is ever as simple as it is advertised to be... So comparing "grass-fed" and "non-grass-fed" animals can get complicated. As to whether or not we need meat, the answer is no, but then we have to find nutritious substitutes for meat... Which can be done, but takes some reading and research.