1.should they be aloud to? well it depends... if they are responsible, probably.
2.Is there any advantage? No... i think its a disadvantage... would you like to be adopted, be raised by parents that are the same sex, and go around with them in public, and tell your friends that your parents are gay... it will follow you till you they die... tell who your married to that your parents are gay... then tell his/her family that your parrents are gay....