When I was at university, a really good friend of mine got dumped by her girlfriend.
She never been with a guy before, and after some time so she could forget about her ex girlfriend, she started telling me that she was curious about how it could feel to be with a man.
I took it as a joke, or at least a random thought passing by, and so I jokingly replied that if she wanted, I could help her with that. After a couple times having the same discussion, it ended up that she took it seriously. I was a bit dumb and it took me some time to realize that she was serious.
We had sex together then, only once, but it was really good. We never did it again because she then was a bit lost about all that, but we are still friends. By now, she recognize herself as bisexual and had other guys in her bed. As for myself, I'm still straight but we don't care x)