Unplug
Hi. I'm completely new here, so you may think, "Aaand who's this guy?"
I very much agree with Penny. In my relationship, we agreed to allowing purely sexual relations, as it was a LDR, before moving on to most recently admitting we were fine with having proper relationships, just because we saw them 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 haven't been together that long. I shouldn't think of myself as her primary. This may not last at all. Just settle down before I get any ideas about her being 'my girl.'"
This doesn't 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 boundaries. You may find after a bit that you can start to talk to PNG about her relationships without automatically worrying she'll be there one day and elope the next.