Something I've noticed about SL that seems a lot different from First Life / Real Life / meatspace is this:
People are simultaneously reserved and approachable.
Bear with me on this, for a moment. It seems to be antithetical, but in a strange way it's not.
The reservation comes from a (well-deserved) reluctance to reveal too much info, be it about their meatspace selves, their age, their location, their true gender. To a certain degree, we are all playing a role in Second Life, be it a mirror to our First Life selves or something completely different. Some details may match up, others not so much. This is widely accepted, especially now that the platform is decades old and most long-time SL residents have learnt to put up some manner of wall between their online persona and the details of Real Life that could be ... sensitive.
The welcoming aspect is a social thing - we are all wanting to interact. We glam ourselves up, we attend parties, we do things together, because SL is primarily a way to interact with others. It doesn't matter if the Resident focus is on fantasy roleplaying, building, scripting, shopping, or simply finding someone to be intimate with; the drive is to socialize with like-minded people. So our avatars, our constructed personas, are things we want to present. We WANT people to get to know who we "are" even if that presentation isn't entirely synchronous to our real selves.
SL is an escape, a form of recreation. We literally re-create ourselves in whatever image we choose. Those images, those identities we assume, we want to share, even if it's just a short time thing.
Which brings me to relationships.
I came into SL with someone else, a longtime friend who I've learnt to trust and care for. We don't have a meatspace relationship per se, other than perhaps "adopted siblings" or best friends. However, in online space, we often roleplay as if we were lovers and partners. So while we don't often get into SL together, that relationship is defined as an SL partnership. It's our fallback, our steady ground. Against that, I've discovered that I can indeed have casual relationships with others, be they one-night-stands, affairs, friends with benefits, what have you.
However, not everyone has a pre-defined partnership, and many are looking for that very thing. They aren't interested in casual intimacy, they want something more permanent.
The trick is, how does one allow casual intimacy, but warn someone beforehand that there's not a place at the table, or in the bed, for a long-term commitment, because someone else already holds that spot? Or (as in my case, a polyamourous exhibitionist with a dominant streak AND a penchant for getting into perilous situations) how do you explain to someone that there's a place for them as long as they don't get upset at others being held in equal regard?
This is something I'm wrestling with. I understand that behind every avatar is a person. That person has feelings, attachments, a desire to be loved and cherished.
So, this is indeed a bit of a philosophical and emotional puzzle to solve. I suppose, the best policy is simply to state things up front - there is someone important already present, in SL, and if a prospective playmate can't accept that, they need to keep their own feelings in check, or walk away.