Yep, you basically read my mind. Romancing aliens is acquired but super rewarding in the end. When I started my first playthrough with a Femshep, I was honestly not going to romance her with an alien because they all look very weird to me, but once I started the game those aliens grew on me tremendously. Romancing Garrus and only Garrus changed from a weird idea to a perfect idea. After that I had a male shepard and Liara exclusive playthrough, and then a male shepard and Tali exclusive playthrough, all three playthroughs ended being perfect largely because I feel I romanced the right alien.
Lol I'll never forget how revolted and confused I was every time I saw Garrus on Tumblr before I knew anything about ME. I was only into DA at the time, but because of Bioware and all, the Garrus obsession naturally leaked into my newsfeed and I just couldn't imagine what it could be that made everyone want to romance someone so utterly creepy looking. I even remember telling myself, "If I get around to playing Mass Effect, I know I'm not romancing that guy".
But yes, on top of the amazing personality/characterization/progression/voice-acting, the alien aspect becomes part of the attraction itself. Something too alien, like a Prothean, doesnt work for most. But for the alien humanoids, the charm comes from everything about them that is super different combined with the things that are familiar and intriguing. That's how someone like me went from "what the hell is that??" to "Hello, I have a mildly debilitating obsession with turians"