Marcus let himself get lost in the kiss. He didn't allow himself to think about anything else but the kiss and how calm and comfortable he was feeling with her -- and he was good at pushing thoughts and emotions far far away, so he could do that.
He didn't deepen the kiss too much as he felt there was no need to in this situation. He wanted to show her he wanted her since he wasn't good with words -- even though saying 'I want you' wasn't something incredibly difficult to say, but it sounded like something out of those romantic novels some women like to read and he was like that. But he wanted to take this slow, this wasn't a fling or anything like that. He wasn't sure what it was, but he knew it was something where he wanted to get to know her.