@Mujus, most companies have a harassment policy. In the large majority of these policies making a comment about someones sexual partner or the way they perform sex is enough to be in violation of these terms. I'm not saying that this case falls directly under that hospitals policy, but it could easily.
@all, I am amazed at how many people seem ignorant of what should be common sense in workplace etiquette. You simply don't say things to coworkers that you know, or even suspect, might be offensive to them. Even if pressed, you just don't go into it. I have been asked a few times what my religion is, or what political party I identify with. My reply is simple "Oh, I prefer to keep my personal views to myself, thank you for understanding", quick smile and change the subject. It is on almost every article you can find about proper work etiquette, YOU DO NOT TALK ABOUT POLITICS OR RELIGION. If you do, then you risk this exact type of situation arising.
As for the exact circumstances of this situation, the simple truth is you don't know enough to speculate. Two situations I can think of:
1) The gay worker may have heard from other coworkers that this particular nurse was talking bad about her being gay. She could have easily overheard insulting conversations, etc and confronted the nurse to see if they where true. Just because someone is religious doesn't mean they are inherently a nice person.
2) The gay worker could be an absolute jerk and demanded an answer from this nurse with the intent of getting her to say something stupid.
Completely opposite situations, those and about a million others are possible, turning this into a debate about a hospital being a fault without knowing the details just makes you look ignorant and is proving even to this day that the founding fathers where right to limit the power of the ignorant masses.