I just kinda see Johnny as thinking that the choice is all his. That if he decides he should marry Jayne then they will get married. Realistically that's probably true but maybe Johnny needs to feel that he's got some competition for Jayne. Of course he already feels as if he's competing for Phi with Ripp but I think that's a different situation he knows Ripp and Phi much better than he knows Stella and Jayne.