How is Romeo and Juliet an allegory for genocide

It's not. However, if you really have to try and say it is:

In the play, the Montagues and the Capulets are feuding. The prologue introduces this idea when it says "from ancient grudge break to new mutiny". We know that this feud can turn violent because the prologue states "civil blood makes civil hands unclean". A feud usually means that one family would like to get rid of everyone in the other family. Tybalt confirms this when he says "I hate Hell, all Montagues, and thee." The bloodshed between the two groups has become such a disturbance the Prince gets involved and threatens the family with execution if they fight again. Since all the children of the Capulets and Montagues are dead at the end of the play, one could argue that the genocide was successful. There are no offspring to carry on either the Capulet line or the Montague line as far as we know.

Genocide is the killing of a large group of people with the intention of destroying an entire race or culture. The number of people killed in a genocide is in the hundreds and thousands.