I thought I was straight until my mid-twenties because although I had fun occasionally kissing girls on a night out or whatever, I couldn't imagine myself actually dating one. Then I discovered there were some women I could probably date, and so I wondered for a few years about whether I was straight or bi. Eventually I got the experience I needed to know for sure, and haven't doubted since.
It wasn't too dissimilar to thinking I was aspie but not having that diagnosis yet to prove it. They say if you think you are aspie, you probably are, and I reckon the same applies to sexuality.