Come on guys, let’s not start another waifu war….
*grabs chips and a fresh beer*
EDIT: On a more relevant note:
It's tied to the conversation with Liara on the presidium where she tasks you with contacting Barla Von over the fetch-quest mercs in that system neighboring Tuchanka. If you tell her she should relax more (top of the wheel, which a lot of people choose by default), the Aethyta conversation will play out as though you're romancing her, regardless of whether you're romancing someone else. If you choose the bottom option ("Practical. I like it.") then you won't get the "my girl" dialogue. Try not to panic, choosing the bottom option won't give you any red meanie points.
I would like to add something to this. I still got the "No one messes with my girl" line with even though I chose the "Practical. I like it" line earlier. This was one of my play-throughs where I was romancing Tali. I think it's also tied to how you respond to Liara on Mars and during the one scene on the Normandy in which Liara and Shepard are discussing the Crucible. She asks Shepard how he/she is able to remain calm and focused(Paraphrased. Can't remember the actual words used). I chose the upper left option at the time(partly because of a force of habit(COOKIE!!!), partly because of RP reasons).
In short, choose the bottom right hand responses(for the most part) when talking to Liara prior to meeting with Matriarch Aethyta to avoid the line. Or lock in your LI of choice before as well.