She's not officially your girlfriend yet.
So she can date whomever she likes, and even go on Tinder. Yes, even with bikini pics.
And so can you.
But please shave before the bikini pics, mmmkay?
Seriously, all of the big lead up to officially starting your relationship is just that: lead up.
You're not there yet.
Neither of you owes loyalty to the other at this point.