[adpiucf]

You can always have the chapter leadership bring this up and adopt into your chapter bylaws:

Two gay or bisexual brothers may not date one another while they are collegiate members of the fraternity. Brothers may not date the sweetheart of the fraternity.

Just put it in writing and put it to a vote with the reasoning that while the fraternity is opening and accepting of all lifestyles, romantic relationships complicate the brotherhood no matter the orientation within the relationship. Use the analogy of dating in the workplace. You don't want to make the chapter an awkward experience or divide the chapter in the midst of a lover's quarrel. The guys should be reasonable enough to see this and you are on a big enough campus where there are others they can date. Or if being together is so important to them, one will leave the fraternity. It is a completely fair expectation.