Everyone is different. Some straight individuals may have a mild attraction or curiosity about the same gender that they would never follow through with, or even an attraction to an idealistic fantasy version of the same gender that doesn't apply physically in real life.

I can only speak for myself, but I consider sexual attraction to be the desire to being physical with another person. If I imagine myself naked in bed with another naked person, or kissing another person, is that desirable or pleasurable ?

As a straight male, the thought of being physical with an attractive women is very desirable. It's instinctual, without doubts, and almost automatic. It's hard not to think of an attractive woman as physically and sexually desirable. However, the thought of being naked and physical with another man is repulsive for me. There is nothing there that I would find desirable or pleasurable. It's a visceral reaction that doesn't require thought or consideration. I know that I have no interest in men. In fact, I'm pretty sure that the feeling of another guy's body against mine would absolutely freak me out, probably with a fight or flight panic. There's no choice there, that's just me.