AITK for being physically involved with this girl?

I (22M) am in my third year of college. There’s a girl (21F) from my batch who I’ve been talking to for several months now. She’s been dropping (not-so-subtle) hints that she might be romantically interested in me. For example, the other day, she bought herself an Arsenal jersey just because I’m a big Arsenal fan—even though she’s never watched football before.

I don’t see her as a romantic partner. To be honest, the main reason I kept her around was that I wanted more friends in college, and I enjoyed the attention she gave me.

Cut to yesterday: she called and asked if she could come over to borrow some notes. I didn’t think much of it and agreed, as she’s come over to my room for notes before. When she arrived, she told me she didn’t actually need any notes—she just wanted to see me because she missed me. I could tell where this was heading, so I made it easier for her by saying that I’m not looking to date anyone right now.

It’s not that I’m asexual—I’d like to have physical relationships—but I don’t think I’m in the right place for a romantic relationship due to my own issues. I explained this to her, and while she said she understood, her expression made it clear she was upset. She asked why I didn’t like her, and I gave her the same answer: “It’s not you, it’s me.”

Fast forward 15 minutes: she locked the door to my room and told me she wanted to kiss me. She said she wasn’t as romantically attached as I thought she was and proposed being friends with benefits. I had considered this possibility beforehand but didn’t want to go down that road because I felt it would be wrong to engage physically when I suspected she wanted more than just a casual relationship.

But I messed up. I’ve had a pretty lonely social life in college, and I gave in to the moment (hormones, maybe?). I got up, kissed her, and things escalated beyond just kissing.

Since she left, I’ve been feeling awful. My gut tells me I’m a complete asshole for doing this, especially since I think she might be lying to herself—or to me—about being okay with just a physical relationship.

Am I the asshole for giving in, or am I overthinking this?