In a word, scheduling.
Successful relationships, whether with spouses, parents, bosses, or even children depend on setting and meeting expectations. If I were to call my wife and say "I am going to a business dinner on Thursday night," then more often than not, she isn't going to have a problem when I come home late on Thursday night. If a woman calls her boyfriend and says "We're having a girl's night out on Friday," the boyfriend can plan accordingly. If, however, the boyfriend finds out at 4:30 on Friday afternoon that he won't be spending time with her that night, you might imagine that he would be a little upset. Is it any wonder that our friends and family are put off when they see you glued to your computer all night? Most of us never schedule time to play our favorite games; we just sit down and start playing. It's an easy trap to fall into. After all, the computer is just in the next room, right? The next time you get hit with the "Inquisition" about your MMO habit, try a compromise. Schedule your gaming time the same way you would schedule a TV show, or even a bowling night. Tell the people you care about "From 8:00 to midnight on Tuesday, I'm going to be playing my game." Make it as much a part of your social calendar as "Dinner with the neighbors" or "Date night" or even your favorite TV show. You may also find that it is easier to balance the demands of gaming with the demands of "Real Life" when you can see exactly how much time you spend gaming versus the amount of time you spend taking care of "RL issues" and "building wife faction."
Whatever you do, don't apologize for enjoying your hobby. You choice of leisure activity is just as valid as someone who chooses to sit in front of a television and watch their favorite show, or someone who sits with their nose in a book for hours on end, or someone who spends all their time crawling around in the dirt planting flowers, or anything else that people do in order to relax and get away from their daily lives for a while. As long as you take care of the things you need to take care of, why should you have to justify your choice of recreation to anybody else?