I'm worried you people will think this is a joke because it's not.
He's not sure when it happened. One moment he and Batman were arguing, as they often do, about who's villains were better. The next he found himself being pushed up against the wall of the Bat Cave, and you have to trust him when he says he is totally NOT afraid of Batman because he is WAY more powerful, so it wasn't intimidation that caused the hairs on the back of his neck to stand up. He was just uncomfortable.
"Bruce, my man! What was that for?" he laughed, expecting the other to let him go and laugh with him. When the Dark Knight did no such thing, he became worried. They were buddies, they were cool, they'd even sang a song about it! Surely Bruce wasn't planning on pushing him out of the friendship ship!
"Say that again," Batman commanded, and Superman blinked.
"Bruce… my man… What wa-"
"Not that! The thing before that!" His was voice was loud and deep and echoed across the Cave.
"Um… All your villains do is wear stupid things on their heads and then make stupid puns based off their theme." He didn't see the big deal. He'd said the same to Green Lantern and this hadn't been HIS reaction. Was Batman offended? He had to know that it wasn't his fault that the bad guys in Gotham City were all stupid!
He was about to ask what was wrong again when the other man let out a growl and then they were kissing and what?
Batman's lips were surprisingly soft for a dark, lonely knight. His hands were clasped tightly on Superman's shoulders but he let go when Superman shrugged them off. There was a noise of surprise when Superman wrapped his arms around Batman's waist, and then a sound of contentment when Batman softly threaded his fingers through Superman's hair.
It went on like this, a chaste, experimental thing, for a while before they pulled apart. Superman stared wide eyed at the other, not quite able to meet his eyes. There was silence as they took in what had just happened, and Batman was starting to remove his hands and pull away when Superman said, "Okay."
"… Okay?"
Superman laughed shakily. "Okay, I'll be your big gay crush. Obviously you want my body, not surprising, seeing as most heroes seem to. I'm not even gonna tell you the things Wonder Woman did once she got into my pants-"
Batman decided the best way to shut him up was with a kiss.
You see, I tend to do this thing when it comes to Starkid where I take ships that are mostly jokey and OTP the patootie out of them.