The word definitely isn't exactly 'emphatic' there. It's an expression of certainty.
When placed before think, it's equivalent to 'certainly':
a) I definitely/certainly think I would feel threatened.
This is like saying 'I'm sure that I would feel threatened.' The certainty relates to the speaker's confidence in his position. If you think of the phrase I think as introducing a kind of personal opinion or attitude, the certainty provided by including definitely in the phrase serves to show that the speaker has a clear opinion on the matter.
b) I think I would definitely feel threatened.
I think this is very slightly different. The word definitely here modifies the prediction of feeling threatened rather than the confidence in the opinion.
To put it in a different way: when definitely comes after would or will, this is a way to show a degree of certainty of a prediction. But in sentence a), since definitely modifies think, this is a way for the speaker to show how sure he is about how he would feel in such a situation.
In the context we have here, sentence a) is preferable because it works better to make the contrast between the speaker's attitude and the listener's attitude to the woman's behaviour. The speaker seems to be hinting that the listener is misjudging the situation.
