Because it's your opinion on what Christ's teachings are, and your opinion on how they match up to that. Doubly subjective.
I don't think you're using "subjective" or "opinion" properly. If I think chocolate tastes better than vanilla, that's my opinion. But if I think that an interview candidate will not perform very well, that is my *judgement*: either he will or he won't, and based on the evidence I have, I think he won't. Very little in this life is ever 100% clear; in the end, we have to look at the available evidence and make a judgement call. This is true of whether we believe what someone tells us, or evolution or global warming, or economics or politics or anything. Sometimes reasonable people can look at the same facts and come to opposite conclusions. But that doesn't mean that any conclusion is the same as any other one. Some conclusions are much more sound than others.
So the above definition is not subjective. What Christ's teachings were is a matter of fact. Whether someone's actions match up to it is also a matter of fact. Sometimes facts are not clear, and sometimes people can be mistaken due to poor judgement or poor information. If you think I'm mistaken about Jesus' teachings, or mistaken about whether someone's actions match up to that, you can try to persuade me to change my mind by evidence and argument.
It may be reasonable for people to come to opposite conclusions about whether Jesus would support abortion, or gay marriage, or divorce. But it is absolutely not reasonable for any person to read his teachings, or those of his disciples, and think that writing "God hates fags" on a sign is something he would approve of.
I propose that a reasonable, objective and widely-agreed definition is: A person who believes that Jesus Christ existed and was the son of God.
That's subjective too (by your definition of "subjective", which seems to be "requires a judgement call"). To "believe that Jesus Christ existed" includes at least some parameters for what this "Jesus Christ" was like -- and if "what Christ's teachings are" is in part a matter of judgement, then "what Jesus Christ was like" is also a matter of judgement. Furthermore, do they actually believe that Jesus Christ existed and was the Son of God? We can't see or measure the internal states of their minds; we can only tell what they believe by how they act. And in my judgement, they certainly don't act like they believe that a man like the Jesus Christ described in the Gospels was the Son of God.
Unless, of course, you mean "A person who has the phonemes J-E-Z-Uh-S attached to some idea, no matter what that idea is." In which case, your definition of Christian is not very reasonable nor very widely accepted.