When they are both online, they are both constantly talking and flirting in it, making it impossible for anyone else to get a word in or have a decent conversation. I really like both of them, making it difficult for me to just butt in with a "get a room" without sounding like a dick. When they are alone, they are still just as great as they always were. It's only when you put them together does a problem arise.
In WoW, you really have to go out of your way to put the P together with the DA. There are many ways of communicating privately: /whisper, /say in a secluded spot, in-game mail, a private Vent server or channel, and so on. Thus, I suspect that your couple enjoys the attention they get by using guild chat for their flirtation, or perhaps just the thrill of flirting where everyone else can read it.
It's not unreasonable at all to assume that other people are bothered by this sort of thing. It might be fun to see them flirting once, but if it's happening every time they're online together, you can safely assume that most people in your guild have had enough at this point.
As an officer, dealing with this situation doesn't have to be a big deal. Privately ask one player or the other if they could take their personal conversations to whispers. I've had to do this from time to time and it's best not to treat the issue like it's a huge problem. That only feeds into their desire for attention, especially if they subsequently feel "persecuted." So the less you say about it, the better off you are.
You might not be able to get away with just that, however. They may ask you why you're making this request. In that case, you can further explain that, while everyone is happy for them, their conversations in guild chat are distracting and sometimes make people feel uncomfortable.
During this conversation, under no circumstances should you use words like "inappropriate" or "gross." Your role here isn't to judge them but to look out for the best interests of your members and clean up your chat channel. Focus on the effects of the behavior -- such as how their conversations make the use of gchat more difficult for others -- rather than the behavior itself.
Hopefully at this point, they'll show respect to their fellow guildmates and take their flirting to whispers. Since they are long-time members, they should know what your guild chat is normally like and how they have changed it.
You can still expect an occasional flirtatious comment. Still, as long as they're not dominating guild chat with extended "private" conversations, you should consider your mission accomplished.
If you are not an officer, then the situation is a little bit more complex. You can make the same request without the authority of an officer rank behind you. In this case, it's more like asking for a favor. Or you could ask an officer if they would mind saying something to them along the lines of what I recommend above. Which course of action you choose depends on how close you are to the couple and how comfortable you feel taking an active role.
People who are in a relationship can sometimes be blind to the effect their behavior has on others. In most cases, just pointing it out -- politely -- is enough.
Send Scott your guild-related questions, conundrums, ideas and suggestions at firstname.lastname@example.org. You may find your question the subject of next week's Officers' Quarters!