Clark is ashamed of wanting Bruce so much, and when he has sex with him, of how much he likes it. Maybe Clark is something of a prude, maybe he has some internalized homophobia, maybe Clark has a sneaking suspicion that he's only being used by Bruce Wayne™ for sex.

Any way, he "gives in" to Bruce's advances and fully expects to be fucked hard and put away wet, little more than a one night stand. Instead Bruce just goes on seducing him, giving him amazing orgasms and being sneakily thoughtful. Gimme all the misunderstandings and needless angst, please! :D