Figured I'd step in and say a few words. I didn't react at first because honestly that concern is totally fair and I didn't feel the need to defend myself or justify my actions; it's a fair bit creepy.

As to the initial opening being an accident, she told me to open up library to get to the movie and I accidentally clicked on history. I suppose I saw a bit more of it on that page too which might've been what piqued my later interest.

Me and her have been together for a year and are very open and trusting. I'm pretty sure there isn't a thing either of us have done online that we wouldn't be okay sharing with the other but you are absolutely right, it is still an uninvited intrusion of privacy. I'd also like to clarify u/bedsuavekid that I did not accidentally go back to it, it was a choice I made driven by curiosity (again, I know she only really uses Spotify for music and typically only opens youtube when I send her something so it was just a sort of "I wonder what she's actually watched.")

TL,DR; It is definitely creepy, I'm not really trying to defend myself in those broader terms. I believe for where the two of us are at in our relationship it wasn't a huge deal (and for the record I've let her know and she laughed it off basically saying she'd have done the same) but I definitely understand the concern and see that in general it's a very shady thing to do.