User blog comment:B214/Talk About What You Want! 2/@comment-4547267-20140710051636/@comment-1359322-20140710062926

That's not the full and exact reason, yes Venelana's scolding might change her thoughts a bit, but it's also because Rias herself has matured since.

If Rias hasn't matured a bit since then she probably still won't allow the girls to be with Issei. But just like how Rias wanted someone to accept and love her for all she is and not only as Rias from Gremory, she needs to be able to do the same. If Rias refuses to accept all of Issei like what Issei did for her, their relationship wouldn't last long. Issei is the type that prioritizes other people's well being over his own after all.