Well, in particular, I'm talking about the first episode, where we get the option to choose what kind of relationship we have with Aeran.

If I chose the obviously romantic option, would it "lock" me to him as only character to romance from then on? Or will I be able to romance others too and then decide which one to pursue?

There are no romance locks in the game yet. 

Choosing Aeran's romance flag sets the continuity for your player character having feelings for him. This is necessary ONLY if you want to pursue any of his romance content. It does not lock you out from romancing or pursuing other characters. (For example, you can flag his romance in Episode 1, but do an intimacy scene with a completely different character in Episode 2. This may have consequences for your relationship with him, but it is something you can do).

Oh, got it! Thanks!