I think there are a few considerations that may influence your situation. There is Your age and Her age. There are Your intentions for relationship with her: You've said you expect an exclusive relationship; but do you expect it to lead to marriage? Do you expect it to lead to cohabitation before marriage?
What's going on in today's college and work-worlds allows greater freedom for alone time between friends of the opposite sex--and one of the driving forces behind that "going out As Friends" category is the need that large numbers of Children of Divorce have to keep alternative sources of warmth and love available to them, since they're unconsciously SO wary about any romantic relationship eventually descending into divorce/breakup. (I wrote a doctoral research dissertation on the love-styles of children of divorce in the 1990s.) So they keep all their "old friendships" alive. I don't know if your girlfriend is a daughter of divorce or not; but even women and men who don't have the adult children of divorce's nearly universal (unconscious) fear of divorce may naturally like the warmth and support for their self-esteem and attractiveness that cross-sexual friendship brings.
It's reasonable that it could take some months for anyone to make the transition from the habit of spending regular pair time with opposite sex friends to reserving such exclusive time for their love partner only. I would expect that transition to occur some time between age 23 and 30. But it's also very related to both partners' intentions in their romantic relationship. THREE years is about the end of the normal biological duration of passionate love--unless the lovers spend a lot of time far apart from each other. There are many factors that can keep a romantic relationship from leading to marriage between the 18 months of courtship that is the commonest period in USA and the 36 months that can often be the beginning of a downward slide in relationship satisfaction, even tho neither partner may notice it at first--because the neurohormonal fuel for sexual & emotional passion is beginning to reduce. I don't want to overgeneralize here, but it's always possible that partners like yourselves aren't free enough to talk about your ultimate wishes -- such as a woman not bringing up marriage, because she's afraid of feeling rejected if her boyfriend doesn't want to go there any time soon. So it's possible that if one of you wants to be sure that you're headed for marriage, but the other doesn't show any signs of wanting that level of commitment to a future life together--and you aren't SAYING THE WORDS to each other, then the one who wants more may be secretly expecting that the relationship is going stale. So the incentive to put "all of your erotic energy" into one person retreats and a holding-on-to-what-you've-got pattern takes over instead.
It's actually more common in America for young men from 20 to 35 to be reluctant to marry than for young women, because men like regular sex and love but worry about the burdens of financial responsibility that come with marriage, and the probability of children. So if you're NOT talking marriage by year 3, but expecting exclusivity, then you are in effect opting for a 20something "trial intimacy" lifestyle; so it's not too different for your girlfriend to be opting for "going out As Friends" as her own 20something continuing lifestyle. That's why your age and the trajectory of your romantic relationship are such important considerations.
Basically, we humans are affected by a biological "mating dance," and if we try to slow and postpone its procreative outcome for too long, it's likely to go flat--so unless both partners are much too involved in their careers or wars, crimes, tragedies or addictions to prioritize their feelings for each other, one or both of them is likely to jump ship and find somebody else with whom the juices are flowing fresh again, and "new love chases away old."
So I suggest you examine yourself for any possible fear of marriage and financial commitments, and you also think about your girlfriend's feelings about the same issues. If you perceive all green lights on these fronts, then you can proceed to assert yourself for the exclusivity you want. I'm not arguing that it's morally right or wrong to want what you want, even if you're not willing to approach marriage any time soon. But having an earnest discussion about this with your girlfriend might clarify for both of you whether you're experimenting with romance or also both moving willingly towards marriage despite any normal fears either of you may have.
I hope you'll respond so that I can tailor my knowledge (from teaching 20somethings about love relationships for 21 years) more accurately to your present mindsets.