Now that I have finished all 158 pages of this thread... took a while, I will be on topic (Though I still see the reapers going for Liara/SB as I stated above)
For me I couldn't get into Kaiden at all, so Liara was my choice for my first femshep (Cathryne is a paragon vanguard). I played ME1 not long before ME2 came out, or even after, don't recall, but before LoTSB came out. For me getting to Illium was like a serious slap in the face. The -only- reason Cathryne didn't move on was because none of the new female LI's interested me at all. Thane almost got me but he is to wrapped up in the past for Cathryne who has to live in the -now- in order to keep hope high.
Back on topic: Liara on Illium before LoTSB it was easy for my manshep Marcus to move on A) Because Tali is adorable and anyone who doesn't hug her when she finds her father is a monster, and

Because Liara was -clearly- not interested in pursuing things right now. Liara knew/knows what limited time Marcus had/has and still pushed him away. Just like Ashley had, though for different reasons. It also promoted the idea that Feron was now her driving focus and alluded to their being more there that Liara had 'moved on'.
Two years or not it was rough on him which is why when Tali at least opened up to Marcus he fell for her.
Now I just recently got LoTSB and am giddy for Cathryne as that was beautiful for her but for Marcus it wasn't renegade, but it was business. Liara had seen to that.
What I'm trying to say summed up is this:
Don't be so angry at those who tossed Liara for a new LI, the way Bioware wrote it before LoTSB came out it was an EASY thing to do for some shep's.
Now after having played through LoTSB with Cathryne I played through ME1 and 2 again with Mara, who is in Adept and Liara the whole way through. Lair has made it hard for me to pick which is my 'canon' shep, as it was firmly going to be Marcus moving forward but now Mara might become it. I am hoping the third DLC will allow Dominic to finally reconcile with Ashley though.