Hi completely new here so you may think "aaannndd who's this guy?"
In refference to penny I do very much agree. In my relationship we agreed to allowing purely sexual relations as it was a LDR before moving on to most recently admitting were fine to having proper relationships just because we saw it as less of a danger to our own.
The best advice I can give to LAM is take a step back unplug and tell yourself "We havent been that long dont think of myself as primary this may not last at all, just settle down before I get any ideas about "my girl". " This doesnt have to be true but just putting yourself in that mindset for a moment can let you think a little more calmly and not worry so much about laying down boundairies. You may find after a bit you can start to talk to PNG about her relationships without automatically worrying she'll be there one day and elope the next.