Without knowing much about the nature of your presentation, I may have a helpful thought or two. First of all, I assume the presentation takes the form of a series of powerpoint slides or handouts or something, correct? And you are expected to be the presenter as a value-added for the material, correct? (If just distributing the materials were enough, I assume no presentation would be necessary.)
So, what I would do is to go ahead and distribute the handout / flip through the powerpoint slides. Think of things you can say with regard to this topic that you do not disagree with, for example, “X Company has a policy of nondiscrimination / no bullying, and I just wanted to take a minute and reinforce that policy with you all.” That’s the key that makes this whole thing relevant to the workplace - the culture where no one is maligned for a sexual orientation that they can’t control. All the other stuff about supporting gay marriage and what not is not really relevant, so you can gloss over it and just reinforce the core points. Unless part of the gay marriage material is to inform the employees that your company recognizes benefits for same-sex couples. Again, just telling them that these benefits exist is not wrong on your part. It is the company that has structured their benefits this way.
In all likelihood, the perception of the audience will be that the material is coming from the company and not from you. The piece they WILL associate with you is what you say and how you present the material.
Apologies if most of the above is not relevant, but without more specific information I can’t offer any better advice. I was just thinking of my experience with my company, who loves to throw very technical power-point presentations at us and say “present this!” so I know from experience how one can take certain … liberties with pre-prepared material.