If there's a question there, it must be rhetorical. As you stated, you're going to hug one of your best friends and joke around if you want to. You may not like the drama or hard feelings that result from your ex's girlfriend, but unless I am misreading your post, you've already kind of made up your mind that continuing your tactile dynamic with your ex will be the priority here.
If you are asking how to respond to her, you might start by asking her why this is so important to her, and sharing with her that touch is something you and your ex have always shared and it's important to you. Ultimately, I think you'd best give her fair warning that you do intend to continue touching the ex when you feel so inclined. It's the truth, and it gets the drama over with which is just as well.
Unless something she tells you convinces you to soften your position? Do try to be a sincere listener while she talks, don't just use what she says to try to build up a good retort.
Probably not gonna be an easy situation to work through. Sorry about that.