Basically Mark sought of sees her in a phyical way as she keeps imposing herself in his life and he grows close to her as he grows further apart from vanessa, some see it as peadophillic but you can see a connection there. Juno feels it (you tell from the lipstick application) and he wants it when he asks how do you think of me"
i hope you find this helpful, if not i'll have another crack at it, but i'm studying Juno for my A level coursework and i kknow it fairly well now
I think both Mark and Juno enjoyed each other's company and it boosted their spirits to have each other. While I think Juno wanted to appear adult and attractive enough to Mark she didn't actually want anything physical with him. I don't think Mark wanted to take his relationship with Juno any farther than a flirtation. I do think she helped him realize Vanessa was not for him.
I always wonder the same thing. Our sympathies, i think,are suppose to shift between Mark, Vanessa and Juno as the movie goes on. They do get close and as that happens we begin to see how inappropriate Juno is behaving eg, showing up at the house uninvited. Then we realise that Mark wants out of his marriage and he takes Juno's attention "personally". I think when Mark tells her he's leaving Vanessa, Juno finally realises whats been going on. Earlier on in the movie Brenda warns Juno that she cant just show up to their house and stuff. That they're married and it's a strange situation.