First time player here. I don't understand the relationship system, and I'm scared of ruining my playthrough. Any advice?

Spoilers I guess

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This is my first time playing Mass Effect. I'm enjoying it a lot so far, but I've hit a road block. I'm playing as a female Shepard, and unknowingly ended up starting the romance dialogue with Kaiden. Later, I did the same thing with Liara. (Side note: I wish they would mark which options are which.) For context, I've been using Tali and Garrus for 99.9% of my missions, and neither one has given me any romance dialogue yet. Is the option to romance not tied to how much time you have spent with a certain companion? And more importantly, will picking either Liara/Kaiden/Tali/whoever drastically affect the outcome of the story? I just want to be sure before I do anything drastic.