The easiest way to convince your girlfriend to sign is by making it clear that it's to protect both of you, not just you. And make sure to explain that you in no way expect to get a divorce in the future. Don't lead her to believe that you're planning your exit strategy. Explain that this is simply a "just in case" plan.
It's a good idea to bring up the prenup issue early on in the relationship, and definitely before getting engaged, in order to gauge her feelings about the issue.
Ask her what she knows about prenups. Make her understand that it's not you against her; you both have input when it comes to the contract. Make her understand that it's not about her getting nothing if you part ways. Don't leave her with doubts.
Ask her to be logical about the situation. Although this will likely be difficult for her (it is for most women), if she really cares for you, she'll put forth the effort.
You can include a clause about cheating, if she begins to question your fidelity. Keep in mind that if you agree to do this, then she should have no problem attesting that she'll remain faithful as well.
Ask her to get legal advice she will eventually see the benefits of a prenup on her own. Tell her you love her, and that this is just for insurance.
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