Maybe you're a Demisexual. For me, I can experience initial sexual attraction to people, but I never act on it. But my actual relationships have been similar to you: I knew them first as acquaintances/friends, we'd hang out, then I'd develop an attraction. My longest relationship was that way. We were simply friends for some months. Well, she liked me as I found out later, but I didn't pick up on it (typical guy). Wasn't until one evening we were hanging out a restaurant just chitchatting, enjoying each other's company, that I also noticed how attractive she was and started letting my mind wander to the possibilities. And I had a GF at the time! And that relationship developed a similar way, too.