It is and it isn't. While it's certainly not a requirement to keep SL separate from RL, I think for most people it's a good idea, at least for privacy reasons. It also helps with the immersion factor. If people do not want to divulge information about their RL on SL, others should respect that choice. If they do, others should respect that choice as well.
On the other hand, it's important to remember that while the SL world itself only exists inside computers, the people who use it are very real, and should be treated with the same courtesy and respect as you would treat someone in real life. Saying SL isn't RL doesn't make it ok to be an **bleep** to someone (unless it's part of a roleplay, and it's clear to everyone involved that it's part of a roleplay).
It's really whatever you want it to be though. Whether or not you want to keep SL and RL separate is up to you.