Justin didn't quite understand why a simple kiss couldn't just be a kiss without him going absolutely mad with desire for Colin, but he was not complaining in the slightest. Rather than being put off by not really knowing what he was doing, Justin used the opportunity to just be led by instincts in a way he hardly ever let happen elsewhere in his life. He didn't feel like he needed to rely on logic right now, though.
Colin elicited a groan from Justin's lips when Justin felt fingers digging into his waist. Oh, Merlin, that felt...god, that felt so good. His trousers were beginning to feel uncomfortable in a way they hadn't done since the last time he and Colin had been together before Justin had been taken away to Azkaban.
One hand slipped downward, fingers playing just beneath the waistband of Colin's trousers before Justin slipped his hand completely beneath Colin's trousers, hand curving over the other boy's behind. His own hips pressed forward involuntarily against Colin's pelvis and a cry of pleasure broke out against Colin's mouth as he did so.