I'm pretty new to java too.. but I think you will need a name, gender and age input prompt for both couples. Than an if else statement to determine if age is < 18 deny else print name, sex, is at a legal age to wed.
edit:
I'm not sure about why gender should matter..well I suppose you have to include the state in your prompt than, since same sex marrige isn't allowed in certain states. If so you would than need to find out which states allow it and make a list your program will use to screen thier state input for acceptable or unacceptable if they are same sex.. don't ask me how I'm new lol good luck.