So when I am dating a girl regularly I have to specifically point out this....?

Maybe I am more old school than I imagined but a platonic friend of mine keeps leaning on me I have to specifically tell this girl I am dating that I'd rather her and I only saw just each other, as if I cannot trust her and I am certain of 95% of her whereabouts at any given moment and etc; and feel I can relax and its not an issue. Plus her actions and behavior don't point to any other man in her life but me, so I feel I don't need to reiterate the status quo. This girl I am seeing and I talk and txt all the time and we are always thinking of more things we could do together, but still my platonic lady friend swears up and down I must come at this girl with such an ultimatum over something that already seems to be the case. Who is right, I'd like to think I am already at where I want but this girl seems to make me think she will keep dating any man under the sun until I ask that she doesn't. If this is what I still have to do to ensure this fine, but I think this girl is not the best one for advice is she is a bit scandalous and is probably using herself as a frame of reference.