lol interesting thread, I have to agree with Cynic and Carbon, however it does sound like she isn't totaly opposed to it, just coming up with excuses not to.
There could however be a number of reasons for this, either she does want to do it but wants further reassurance or convincing, OR she is just being open to it because she actually cares about you so much and doesn't want to push you away by being negative.
If its the former, then sure, maybe you could do more to help convince her, but obviously be gentle and not forceful, however if its the latter, then stop right now. Otherwise, she will end up gaining purely for you and if anything happens to the relationship she'll forever resent you for fattening her up even though she wasn't that into it.
It is a difficult situation and this is why as others have said, its easier to do this with someone who you know is into it, I think the best you can do is have a serious talk with her about it and see how she really feels. Just be warned that this could result in the ending of the relationship if you don't agree, although if this is important to you and not her, that may be wat needs to happen anyway.
Good Luck!