EVERYONE is SL is "serious", just maybe not about the same thing(s) you are. Just as in RL, people in SL are serious about their own agenda, which may or may not have anything in common with your agenda(s).
My advice for anyone in SL looking for a "serious" relationship is, if you have the physical ability to shut off the computer and get out into the real world, do so ASAP. Almost by defintion, SL is a place where people pretend to be who and what they are not in RL, and such a starting point is diametrically converse to what should be the foundation of a stabile, healthy RL relationship. I'm not saying it could never happen, but the odds are greatly against it.