I also think it's interesting to see the different reactions to the description. And sometimes I think it's really difficult to just describe something in words, you almost had to be there, to get the "vibe" and see how other people were reacting. I don't think I'd go so far as to call it "immature" or "inappropriate," but as I was reading the description I was kind of rolling my eyes at them.
Even if the PDA isn't "graphic" (however you define that), if they are exclusively paying attention to each other for significant amounts of time, instead of being open to interactions with others, I think that becomes rude. I can see how that would be really difficult to regulate when you're part of the couple though--it's hard to see your behaviors from the outside.
I am also guessing that Zoe and Zac are still in the "lovey-dovey" phase (8 months, after a long time of being lonely) and may have calmed down considerably by the next time you want to invite them to your parents' house. You could do a test run by inviting them somewhere else first and seeing how they behave.
Or, if you wanted to mention it to Zoe, maybe something like, "Yeah, it seemed like you guys were really into each other. That's great, I'm really happy for you. I just wish we would've had more time to chat at the party, but it seemed like you guys were off in your own little world." Okay, it's kind of a soft opening, and someone could easily take that as a total compliment, but maybe someone else can come up with better wording. I do like the "off in your own little world" thing because to me, that would be the rude behavior--they could have been sitting on opposite sides of the counter, not touching at all, but if they were staring into each other's eyes the whole time, they were still rude. But, maybe that's not an accurate complaint in this case, if the mom was more uncomfortable with the actual physical contact.