We'll I'm currently dating a soror. In my case, we don't have the sorts of problems that were mentioned here because she did not pledge here nor did I. In fact she was Zeta when I met her (I was not a Sigma).
The relationship has been good and fulfilling.
So, I'd say that it is okay to date within the Blu and White, just be aware of what is important to you as a dating couple and what is important as members of the blue and white.
Many times and again, I have gotten into trouble by not separting the two. (I.e. I get a little too fervent about the frat.
)
Six_Three_Sigma