People should get to know each other before sharing their feelings. It would save a lot of heartache and damaged self-esteem.
As for Cancer men or men in general, I think it's rude and immature to just ignore someone because you think a woman is falling for you too fast.
Be an adult and let the person know it's too soon for you. It might hurt her feelings, but at least it's better than disappearing and saying nothing and hoping that she's a mind reader.