The obvious way out of what? "...the previous night held something distasteful...arising out of the history of their association." "His rejection of her in the parlor had been the only sane course." Marrying Demelza was the way out of the shame that he feels as reflected in Shakespeare's sonnet that keeps running through his mind: "the expense of spirit in a waste of shame is lust in action...past reason hated".
In the parlor it's the memory of his mother, "Grace", that stops him from progressing with Demelza. Then in his bedroom there's the memory of his father calling him a "Young prude!" for restraining himself. The result is that Ross manages to follow both of these influences in his decision to both sleep with Demelza and then to marry her. The Joshua in him found her desirable and the Grace in him honored her with his name. The result is the character that we come to adore, Ross Poldark, the man who flouts convention and marries his maid instead of just sleeping with her.
Finally, there's also the impact Demelza herself made on Ross since day one, "He suddenly found that...if she would not desert a friend, neither could he."
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