Marilyn Monroe and James Dean never dated. They never even met each other when they were alive. Monroe was several years older than him. While Dean was starting to establish his career and status in Hollywood, his life ended abruptly. Monroe died several years later as well. They each lead their own lives.
However, both Monroe and Dean are considered icons in Pop Culture.
Both died at very young. They are often depicted in art indicating that their beauty and tragic lives.
Monroe and Dean allegedly met once during the premiere of his debut film East of Eden(1955) and apparently disliked each other over 'Hollywood differences' (she enjoyed the spotlight and he hated it). They would've made an attractive couple though.