Obsessed With Watching My Partner Undress

so i met my long term partner in college, we've known each other for a few years now. we're both pretty chill, but lately there's been something i just can't shake. i have this weird thing where watching her undress and get ready for bed makes me feel so turned on, even though i already know she's mine. it feels like i'm being someone else or living in a fantasy world. why am i doing this? is this okay?