Redhawk Caldera i've seen this one before.

She saw the way her words hit him, saw the shock wash over his face along with the confusion. Let him ask how she knew, do it she dared. But he didn't, instead he pulled himself together and posed a perfectly normal, reasonable question. Why did it matter?

Fin felt her eyes narrowed into slivers. Why did it matter? Why did it matter? Could he really be so stupid, so blind? She felt her heart aching as though each word was another knife stabbed deep within it. The fact that Fin hadn't even realized it until she saw him mount the black bitch was far from her mind. He should have known. The fact that he had no idea, that he'd just done whatever his stupid dick told him to was all the ammo she needed to be pissed as hell at him for what he'd done.

"I suppose it doesn't really," Fin replied, bitterness upon each word, "I just thought better of you I guess. My mistake, apparently." It was ever so hypocritical. It wasn't as though she hadn't had any one time flings in her lifetime. Hell, she'd had a bunch. But still, she hadn't done that knowing someone had feelings for her. And he should have known, Fin was still determinedly believing, he should have known.