And when that question is asked, deny everything.
I know it seems hard and contrary to every human (or at least male) instinct, but if a girl in-game proposes anything that smacks of a relationship, and you think you might want to pursue her seriously, DON'T DO IT! For the love of God, yank your Ethernet cable out of the wall if you have to, roll a new toon of the opposite faction and level it for a few weeks, even commit a misdemeanor and spend a month in jail if you have to. (I hear public urination is all the rage this time of year, incidentally.) Just do NOT try to make a WoW relationship work!
If I seem somewhat passionate about this maxim, and it almost looks like I'm speaking (well, typing) from personal experience...well, I just might be. Believe you me, it is twice as difficult as making a real-life relationship pan out, with just a fraction of the benefits. For one thing, there's no going out for a quiet dinner (no, eating a Tough Hunk of Bread in Teldrassil together does NOT cut it), or watching a movie afterwards (you can try sending each other YouTube links, but trust me, it's not the same--and the popcorn tastes strangely like carpet). You can't look into each other's eyes, exchange sweet nothings, and cuddle (although if you've ever gone to the second floor of the Stormwind Inn, I bet you've seen a few couples trying their hardest to make something happen on the bed). As for, ah, more intimate accoutrements, suffice to say that a keyboard in one hand and a, uh, "mouse" (wink, wink, cough, cough) in the other, is not a good surrogate for a healthy sex life. (Not that I would know what does constitute a healthy one, unfortunately.)
And try as you might to overlook it, there will always be a kind of competition between the two of you: who's the higher level, who has the better gear, who's got a higher rank in the guild. You might laugh at such "virtual" tokens of station being capable of wrenching two lovers apart, but if the game was enough to bring you two together, you better damn well believe it's enough to rend you apart. Not to mention all the difficulties inherent in traditional long-distance relationships: increased risk of infidelity, little or no face-to-face interaction, astronomical phone bills, etc.
If nothing else, think of this poor, heartbroken Paladin who once loved a level 60 Night Elf hunter (whose names, both in-game and real, will be kept secret to protect the skanky). Think of all he did--in real life--to salvage the relationship, and think of how devastated he was to lose her to a level 38 Human Warlock. (Yet another reason to hate those goddamn locks.) If you don't want to end up like me, then for God's sake say no to WoW relationships.
Best of luck to you Casanovas out there...