According to Mordekhai (Kiddushin 533, to Kiddushin 65b (page 4a in the Vilna Mordekhai; he cites Rabbenu Barukh), a couple who lives together, even without any legal ceremony requires a get upon separation.
This is an oft-cited opinion, but in reality, he is in a significant minority. Very very few poskim agree with him about this.
Accepting this idea means that a couple who wrote a ketubah and did the seven blessings under the chuppah would be fully halakhically married, without a ritual of acquisition. On the other hand, the woman would still technically be considered acquired–it’s just that the cohabiation (and presumably the conjugal relations) effect the kiddushin rather than the passing of a ring.
ADVANTAGES: No ritual enactment of acquisition, fully halakhic marriage.
DISADVANTAGES: Acquisition happens nonetheless, a get is required, this really is a minority position and should not be adopted without serious consultation with a qualified rabbi.
This Mordechai might be used as a “Snif Leheter” under certain circumstances… Example: A Cohen cannot marry a lady whose father is not Jewish. (E.H. 7:17). However, if married they need not be divorced. (Nosei Kelim Al Asar.) Now, if a Cohen lives together with a lady whose father is not Jewish there is the possibility that they are already married, which would allow them to continue living together, and may even permit a proper Kiddushin (misafek?). A competent Rav MUST be consulted, however.
Although, the glaring problem with attempting to use this Mordechai Lehalacha is that any time a live-in BF and GF separate they would need a Get, otherwise her future children would be (safek?) Mamzerim, not a good outcome…