but it's also telling I think that the boy in question was the one who was least likely to want to take things to the next level (very socially awkward and religious with a purity ring), so I wonder if I liked him because there was no chance of it becoming a real thing

Feeling attracted to "unavailable" partners has nothing do to with your sexuality though. It can be related to your attachment style, insecurities of yours, etc etc. I'm a lesbian and I've noticed over the years a pattern and I usually tend to attract emotionally unavailable women. If you felt attraction towards that guy, you're not a lesbian.