Bruce always buys the gossip rags so he can read about how unsavory they think he is. Part of him is repulsed but another part of him gets off on it.

One day at Bruce's place, Clark picks one up out of curiosity and is all, "Bruce, this is disgusting. They make you sound like a slut." Cue Bruce provoking Clark into an argument in the hope that he calls him more demeaning things.

Whether they're in a relationship or not is up to you, as is whether they fuck or Bruce just jerks himself to it later.