Online RPGs are a strange world. I've been playing one of the very first ones created for 10 years, and it has been around for 15, (no graphics, just text!). I will be the first to admit that there is a downside to playing them, but generally most of the problems people have with them are not the games themselves but WHY they are playing them.
If you are playing a game as an escape from reality, then you (or your husband in this case) need to do some serious soul searching. What is the person escaping from? Is it just a small presure relief or a major time sync that is getting in the way of a normal life? It is possible to spend day after day in front of a computer, typing away, while the world turns around you and you miss it all. But it isn't really the game that causes the problem is it? The game is just a facilitator. In fact, I'd wager that there are some people who post on these boards who do so as a form of escape as well, and there are probably some who spend way too much time doing so. And yet getting out of the game might be the easiest thing to change and a good first step toward a better life.
Gaming or posting, you can turn yourself into a CPU potato and ignore much more important things in life, even your health or your family, (which in a fair number of cases is exactly what people are trying to escape from).
There is another thing too, and that is addiction. Anything you do a lot of can become an addiction. RPG gaming can be an addiction just as much as smoking, especially when it is a form of escape and it effectively gives you pain avoidance and a sense of euphoria all in one dose. Sounds like a drug doesn't it? All things in moderation.
On the in-game marriage thing, I have to admit that the first time I heard of it I thought it was nuts, but you have to keep in mind that it is a game. I play an RPG character who has been married for a good number of years and I have not one single moment had a romantic attraction to the person who PLAYS his "wife". Is it weird? Probably at least a little bit. Is it hard for people who don't play RPGs to understand? I'm certain of that.
I think the bigger issue isn't that he's got some fake character telling some other fake character that he loves it/her, but it is really that he is escaping from his marriage with you. If that is the case, then I think Azzy isn't too far off the mark. You two need to sit down and talk about it, specifically about how it is affecting your marriage.
On the other hand, if he's talking or txt messaging with the PERSON who plays the character and telling HER that he loves her, that's a different story. If that is the case, cut him off at the knees.
The number of people who spend hour after hour playing a game is growing and I'm not so sure it's healthy. Of course, largely they are replacing hour after hour of TV watching with gaming. I'm not sure which is better/worse. Probably a walk in the park or a trip to the gym, or a bible study group!
I don't think a small amount of escapism is unhealthy mind you. Afterall, we do go to the movies for the same purpose. But if he's constantly in front of the computer playing his game, then he needs to cut back or quit.
Best wishes.