I'm afraid I'm with YGirl in saying that your approach is mature and professional, and I'll happily jump into the assumptions and stereotypes on this one.
You need to make your assessment about the field that you're in. I've not had the pleasure of getting involved in grad studies, so I don't know how the politics work exactly in that. My own feelings about academia is that while the student body tends towards the liberal, those at the top of the ivory tower, the profs and administrators congeal into an organization that is similar to many others of their age and size. Due to policies, public pressures, tradition, politics, etc. it's an organization that isn't the most responsive to change. Working in a not so responsive organization, I do know a little bit about that.
I will assume for the moment that you have already assessed that those in your field are not likely to be understanding. Or that being out of the closet, or being involved romantically by proxy to co-workers is going to cause issues with your career either during your studies or afterwards. In which case I do not see any issue with requesting your partner look elsewhere for the dating pool.
Does it reduce the numbers? Yeah...12-30 people? How many grad students are we talking here? Out of what? A couple thousand students and staff on campus? How many in town? Tens to hundreds of thousands? 6 Billion worldwide? It's not a great sacrifice to the pool numerically. The better question might be how committed is your partner to allowing you to develop and pursue your career?
At the very least it would be reasonable for the time being until such time as you are either established in the field, and/or you have had time to determine how such life/lovestyle choices would be received in the community of fellow professionals. However, while you are starting out...and while your ability to get a degree is still under the prevue of those who may be able to block your progress due to personal opinions about your way of life, it may be a risky time to be doing things which could rock the boat. If you think they’d so something like that, then your partner would be better to respect your wishes on this.
My 2% of $1.