SL stays SL...yeah, right...
Although, this decision has to be taken again and again if you stay in SL for some time. You meet people you like and soon you catch yourself wondering as to where they are at in real life and who you are actually talking to. This is human nature, I guess. Even the people who have basically logged off from RL permanently have a hard time denying that there is still a human being in flesh and blood moving around that cartoonesque figure on the grid. So most of us will start to share tiny bits of our Real Life here and there. More so if you engage in close friendships or even relationships.
I don't go around telling everybody who the person behind Moe is, either. I refrain from putting a real picture of the rl-me in my profile. So I guess, I like to be candid to a certain extent myself.
Being true to myself, I had to admit early in my second life, that Moe in a way is just an extension of my real self. I might not be a 6.3 hunk with a fancy wardrobe in real life. I might not like to play dressup as much, running around as a dragon or a wobble in town. Yet, Moe shows most of my basic character traits, shares my values and opinions. He cannot engage in things I detest in real life.
Some people might test their borders to some extent, stretch their beliefs as to see how far they would go if able to hide behind their second self. And that's a good thing, too. For some SL might be a bit of a valve to blow off some steam - or an experimental territory that helps them to expand their personalities. And this might even have a positive effect on their first life. I have heard many people say so.
I am sure, though, that most people will, with time, find more and more of there real selves in their avatars. Even the traits they don't like about their real selves will show.
The good thing about that is, that if you are attentive, this also can be somewhat of a mirror to yourself. It has been to me.
Once one has been in Second Life for sometime, one will notice that also this seemingly endless grid is as tiny as the real world. You will run into the same people over and over again. You will find out that most of the people you meet, know someone you know. And your behavior will have an effect on your second life just as is has on your first one.
And on some occasions you will even run into someone you know from your first life - and chances are, you will give yourself away.
I don't think there is anything wrong about that. You might not know who the person behind Moe really is...and mostly this doesn't really matter either. But if you are friends with Moe, chances are you would be friends with the person behind the avatar also.
There might be an exception if the person behind the avatar is not an adult. Not having found your place in the real world yet will have its effects on your second life as well. But as an adult you will always bring your basic character, your cultural background, your upbringing, your current situation and surroundings into your character.
What I am basically driving at here is: It is fine - and sometimes even needed - not to share everything about the person behind your avatar with everyone on Second Life. But the claim "SL is SL and RL is RL" will never withstand a thorough examination.
My suggestion is, not to spend too much energy on hiding ones real self from the people in the metaverse. The real you will always show. Rather spend this energy on exploring your real self through this gift of a second personality. You might learn a useful thing...or two about yourself.