Conscious of their audience, Sirius deepened the kiss a little more than he might have usually, when they had some place to go, but he didn't draw it out for too long. He wanted to show James that he meant it about Gaius, but he didn't want James to offend Gaius by doing something like counting seconds. And as immature as he's been acting, he just might.
Smiling, he trailed a hand along his lover's cheek. "All ready, Cat. Do you mind if James walks down with us? He says he has trouble going places on his own, but should be all right once he's there. Oh!" He took a step back. "Gaius, this is my old friend James, Harry's father. James, this is my boyfriend, Gaius." He hesitated a little over 'boyfriend'. Did that sound too immature? 'Lover' might have been better, or James might have taken that to mean it was just sex.
"'Lo, Gaius," James said, civilly enough. He eyed Gaius up and down in a speculative way, but kept his thoughts to himself.