I have to disagree with the responses that suspect that this behavior is a precursor to hidden demons and that he might suddenly grow fangs.
Take a step back and ask yourself how you, or most people, learn about their partners sexual preferences, likes, disklikes, and boundaries. We all like to think that we are open and forthright, and 'talk about it'. In reality, we seldom talk. We get to the point of being intimate without verbally discussing details - it is too unnatural to do that. So we ease in and slowly touch, kiss and play. As one partner tries something and s/he gets a positive or neutral response from the other, they learn that the given act was 'ok' or acceptable, or pleasurable. We do this again and again until we learn about our partners preferences, likes and dislikes.
I, for eg, like doing waht your bf does - pull on my partners hair hard. I don't always do it, neither do I start off with that the first time I'm with someone. It is gradual, and over time, I learn whether my partner is into it, or can accept it. If I get no resistance, I assume that it is okay. I'm inclined to think that your bf is making a similar deduction/assumption - you haven't complained, he likes it - so he assumes that all is well. If all is not well, it is up to you to make that clear.
Is it normal? Yes, it's normal. Just do a search on this site for question about why long hair is considered sexy, and you'll notice that many of the answers teasingly imply this as one of the main benefits of having long hair ;) But you are right that it is not 100% common - there are lots of men and women who are into this, and other forms of pain. To be perfectly honest,sex always involves a small amount of pain and pleasure mixed together - doesn't it? So where is the un-naturalness in another sexual act that depends on mutual pleasure-pain responses as long as it is consensual?
If he insists after you tell him that you don't like it, he would be in the wrong and disrespecting you. But I suspect that this could be a deal-breaker for him. So unless you are willing to risk the relationship, I would suggest that you try to find a middle-ground.
- Response by An Intellectual Guy, Male, 26-28, Seattle, Technical