The minhag in Yehuda was to be meyached the choson and kallah after erusin. The intention was, according to Tosfos, was that they have biah, and they had sheva brochos as well. DId they not hold of the birchas erusin and the issur described therein of bias arusah?
The intent of the Yichud is clearly stated to be that they should be familiar with one another, not Bi'ah. Tosfos writes that we take the precaution of saying Birchas Chasanim in Yehudah lest there be an unwanted Bi'ah, in order to alleviate the Isur.
Dov Zupnik
The Yerushalmi (Yevamos 1:5) states clearly that the takanah was for the husband to be boel v'odah b'veis aviah.
Y. Landy
[The Yerushalmi explains that the Minhag in Yehudah began in response to the Hegmon's decree that he takes every Besulah that marries. In order to assure she will remain faithful to her husband and go b'Ones, they instituted this custom, to strengthen her loyalty to her Arus. The evil decree eventually passed, while the custom remained.]