Some consequences yet hanging in the stars,
Shall bitterly begin his fearful date
With this night’s reveals, and expire the term.
Romeo is saying that there will be consequences in what is about to happen and that something is destined to happen. He will find out that night and his thoughts will expire.
If he be married my grave is like to be my wedding bed.
Juliet is saying that if Romeo is married she would prefer to die then marry somebody else that isn’t him.