Is it unfair to begin a relationship with the idea that you won't stay long?

To think "I'll date her for a while but I'll leave when things get serious" (fear of opening up/showing vulnerabilities and weaknesses). You are really into them, but don't see yourself ever being willing to let them in. Is it unfair to them? It's not like ending a relationship before things got serious would really hurt them much anyway.(I say 'she' because I am bi)