it was rather interesting to read your post about pros and cons, because it i think it really highlights the way that men and women think about swallowing. from the sound of it (and this isn't necessarily true), my guess is that it IS about the "tabooishness." and that's understandable. as a woman, i think that, somewhere along the line, we are taught swallowing is demoralizing. from your perspective, swallowing would increase intimacy and closeness, but from hers, it may do exactly the opposite.
it sounds like you've pushed the issue quite a bit. if i were in your position, i would recommend posing things to her in a different way. tell her that you were thinking about the whole issue, and that, while you've thought it could make you closer, you have realized that it could be totally different from her perspective. apologize for not having recognized that before, because, in truth, your thought process has kind of been lacking in empathy. explain to her that no matter what she chooses, you'll love her either way (and hopefully reading this will help you understand her side to the point where you can say that genuinely), but that if she wants to try it at any point, you're open to it. this, i think, will take the pressure off of her. it will allow HER to make the decision without feeling like she has to do it to impress or please you. it will also express some emotion and consideration for her feelings and perspective, which all women appreciate. but if you do this, you can't continue to push the issue. i think that if you pose it this way, it will happen after a little while. try to be patient, but ultimately, you really can't go into it with any expectations. its important to understand that.
in the case that she doesn't eventually try it, its likely that it really is just an issue of taste... and you can go from there. BUT, if she DOES eventually do it, you absolutely MUST make sure to reciprocate her intimacy and efforts immediately afterwards. that is, hold her, kiss her, tell her how much you love her (or even better, WHY you love her), etc. to let her know that you associate INTIMACY with swallowing instead of demoralization or submission.
hope this makes sense, and good luck!