What's the deal with sternum touching?

Ok, so when my sternum is touched ( breastbone, in the center of the chest), I really don't like it. I freak out even if I do it to myself.
I looked it up online, and I found nothing. I've also seen Pretty Woman.

Is it a thing to do to your significant other when having sex? What's the logic behind it? Why am I freaking out about it?
What's the deal with sternum touching?
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