So far my understanding about "Poila jane" word much more popular in Aryan because they were more strict on their culture and did not let their daughter to choose own groom. Instead they choose themselves and made one retual in Marriage " Swayamber", which is copy from past history. Except Bramhin and Chhetri , rest has freedom to choose their groom. So who ever did not have choice of choosing own groom has force to be part of " Swayamber". Those who retaliate and choose own groom. Sita, Bihe garnu agadi Poi khojera gai". The wording is too long , needed a slang or short one. It is apvramsa of " Phalani bihe garnu agadi poi khojera gai" So it turned to " Pahila gai" later it got into janjibro as POILA GAI.