I think there's nothing wrong with getting married in game, so long as the two people follow some ground rules. Generally speaking, I'd only get married to my RL spouse, or a good close friend that I know really well. But that's just me.
1: It's a game. You (as my game spouse) have no claim on what I do in real life, who I party with in the game, and if I decide I want to log off instead of party with you or roleplay with you, don't get pissy at me, I have a real life to take care of, with a real life spouse/ significant other/ family/ chores/ hobbies. If I don't log on when I "normally" do, don't interrogate me on where I was, what I was doing, who I was with. If it's any of your business, I'll let you know.
2: It's a game. Sometimes, I'll prefer grinding XP than roleplaying. Deal with it.
3: It's a game, and in out of "character" areas/context, don't refer to me (in a serious tone :P) as your sweetheart/ love of your life unless I know you really well. Don't make jokes to my RL spouse who also plays how you're going to steal me away from him/her. It doesn't really go over well with either of us. (this is where knowing the virtual spouse really well as a normal friend helps - no wierdness issues about personal jokes/teasing out of "character")
4: It's a GAME, I play it for fun. Sometimes that means I'll roleplay (or not roleplay), be silly, act out of "character", without you. I expect you to do the same. If you don't do the same, don't try to blame me. It isn't my fault. I don't make plans around you, don't make plans around me, unless we both make set plans to do something together.
5: The only reason I'm virtually married to you is that I found you fun and friendly. If you stop being fun and friendly and start doing any of the above, don't expect me to remain virtually married to you at the expense of having fun. It's a GAME!
(Why, yes, I've had problems with this before in many different roleplaying games, why do you ask? :P)