How did relationships/ marriages in the play change history?
There's an extremely strong contrast between the marriages of Brutus/Portia and Caesar/Calpurnia. When Portia begins to realize that Brutus is up to something, she whole-heartedly (and blindly) supports him in his endeavor. Even though she doesn't know what he's planning, she send Lucius to essentially tell Brutus that she's ok with whatever it is and will be behind him no matter what.
In contrast, when Calpurnia senses something wrong, she urges Caesar to stay home to stay away from potential trouble. After a little nudge from the conspirators, Caesar feels his manhood threatened by these worries from his wife and decides to go to the Capitol to proof that he's not afraid of omens (remember that earlier in the play he stated that he's a more fearsome lion than danger itself!). Had the two wives acted in opposite manners, neither Brutus nor Caesar would have been at the Capitol. Brutus, as the loving husband he is, would have stayed home simply to comfort Portia. Had Calpurnia not threatened Caesar's arrogant nature, he may have been more inclined to also stay home.