This is a tough situation, and the dialog about gender in the tech scene this summer has only made such situations more fraught and adversarial than they might otherwise be.
Remember that just as many thoughtful and considerate men are frustrated because they feel like they're being tarred with the same brush as VCs who abused their power to get away with sexual assault, many women are feeling like they're being told they don't belong in the jobs they've been doing well for years, like their complaints that they have to struggle daily against subconscious bias are being dismissed without even being considered, and like if they want to even pursue their careers they're expected to accept everything from subtle disrespect to outright abuse.
You have a choice to make, and your choice will contribute in a small way to whether the conflict - in your company and in the industry - escalates or de-escalates.
The approach I would suggest depends on several factors:
1) prior to this incident, did you have a good, friendly professional relationship with Sarah?
2) prior to this incident, has Sarah demonstrated a tendency to take professional critique or social/political commentary personally, or does she usually take things in good faith (even if she presents counter-arguments)?
3) do you want to approach this issue in a way that's adversarial, or do you want to repair and possibly further your working relationship with Sarah?
If your answers are 1) "yes", 2) "good faith", and 3) "repair", then I would approach it as follows:
First, ask Sarah if she'd be willing to talk over lunch or coffee or even just to take a walk outside. This sort of discussion is best had in a setting that's public but allows for private conversation and is less formal than the office but is not easily mistaken for a romantic overture.
Tell Sarah that you're sorry that you said something that came across to her as sexist. Without trying to justify anything you said, tell her that a) you didn't intend to say anything sexist; b) that although your initial reaction was to be hurt that she would accuse you of sexism, you c) realize that you may have said something that was hurtful to her without realizing why it would be hurtful; and d) you would like to understand what it was about that interaction that she felt was sexist, so that e) you can learn from this experience and be better in the future.
If she's a decent human being and if you're willing to be sincere and more importantly to really LISTEN, this is the sort of conversation you can learn a lot from -- both about Sarah as a person, and about the sort of subtle frictions and obstacles women face in the workplace on a daily and weekly basis.
If you do this right, and if you mean it, Sarah will learn that you're someone who means well and is willing to make an effort to be a good colleague to the women in your office. The next time you say something that perhaps could be taken the wrong way, she'll be more likely to give you the benefit of the doubt and instead of accusing you of sexism, merely point out that what you said might not have come across the way you intended.
You'll get to understand that she reacted the way she did not because she's an unreasonable person looking for excuses to fly off the handle at her colleagues and get them in trouble, but (probably) because she had a real reason to interpret a situation differently than you did, even if she was having a bad day and didn't handle the situation as well as she could have. You'll discover what it was that set her off in the first place; what the (almost certainly not malicious) reasons were for her interpreting it the way she did; and how to avoid repeating such an incident.
An open and honest discussion like this can add a great deal of strength and trust to your working relationship, in both directions.
If you're not willing to be humble and to listen; or if Sarah is not a person you're willing to trust, then I would do the following:
Document, document, document. Write down your take on what happened. Put a date on it. Do not turn it in to HR. If anything like this recurs, also document it immediately. Then, if and when an accusation or confrontation does occur that does get HR involved, you will be able to show them your side of the story instead of just babbling that you didn't mean to offend anyone.
