So far you have not indicated your gender, and that may have a bearing on it.
edit: It has just been pointed out to me by Shoover that gender was noted in a self-comment.
I have not been back “home” in over 30 years, but when I lived there I was not aware of any hidden meaning to a “slice”: a slice was a piece of pizza, and given that you were seated at food court eating pizza, I would normally tend to think that was all they had in mind. However, I did do some digging, and found a few references to sex (not just Urban).
The Urban definition is just too disgusting and misogynistic to repeat here, and sounds like some teen-aged boys participating in a gross-out contest. Certainly the braggadocio would indicate an immaturity of thought, as well as an outsized opinion of their questionable sexual prowess.
I found this conversation at Word Reference.com
In the movie "Mean Girls" one of the girls tells the other
A: Oh, you'll get socialized, all right.
B: A little slice like you.
A: What are you talking about?
B: You're a regulation hottie.
Answer from senior member Chris K.: "Yes, it's a slang term for vagina."
And in the online Slang Ditionary definition of slice…
"a piece of". Whether it be a person (sexual), or a thing, such as wisdom...
So…if you were just a few guys hanging out, I would discount any double meaning, but if you were in a group of girls, it could be that they were playing with your head, and hinting at a hookup.