This is a complicated topic but a good thread. I wonder about it often enough.
My high school sweetheart and I used to fool around a few times with those singles lines, masturbating each other while we had phone sex with strangers. It just seemed like fun experimentation. After we parted ways but still fooled around, she asked me I'd mind if she had sex with the mutual friend who had introduced us but had always been crazy about her, just to see what it was like. I was surprised that I didn't and encouraged her to go for it. Instead, she ended up having sex with another friend of his in the shower and gave me a great blowjob a couple weeks later telling me about it.
The signs about what I was into were always there, but it didn't really click.