I cannot say Willoughby’s initial intent when he asked Mrs. Dashwood for “the honour of calling to-morrow to inquire after Miss Dashwood” (Ch. 9) was to gain Marianne’s affection yet withhold his own. Perhaps at that moment something like flirting was all that was on his mind or he was just curious, perhaps intrigued by the interesting women he found in the cottage. (:D) In Ch. 44 he says:

“When I first became intimate in your family, I had no other intention, no other view in the acquaintance than to pass my time pleasantly while I was obliged to remain in Devonshire, more pleasantly than I had ever done before.”

However I can say if Willoughby’s initial intent was more or less innocent it did not last long. In his first visit (Ch. 10) he lied to Marianne about his tastes, opinions and enthusiasms to gain her good opinion. This agrees with Willoughby’s confession that “I endeavoured, by every means in my power, to make myself pleasing to her” (Ch. 44). Willoughby did not suddenly realize Marianne was in love with him. It was not an accident and it was no surprise. It was his plan. He said “I was acting in this manner, trying to engage her regard, without a thought of returning it” (Ch. 44). He was successful. What Willoughby got from the relationship and why he stayed are the same, they are also to be found in Ch. 44:

Your sister's lovely person and interesting manners could not but please me; and her behaviour to me almost from the first, was of a kind -- -- It is astonishing, when I reflect on what it was, and what she was, that my heart should have been so insensible! -- But at first, I must confess, my vanity only was elevated by it.

Willoughby liked the way Marianne’s rapt attention and her warmth made him feel. It “elevated” his vanity and made him feel good so he decided to assure he continued to benefit. It was his plan (rather calculating) to gain her affection for his amusement and although aware, as he told Elinor (Ch. 44), “To attach myself to your sister, therefore, was not a thing to be thought of” it was not a reason for him to leave. He was, after all, acting selfishly without regard for Marianne as he said: “Careless of her happiness, thinking only of my own amusement” (Ch. 44). You do not appear to believe the enjoyment of Marianne’s company was enough incentive for Willoughby to stay in the country and act as dishonorably he did but I can see it because it is quite intoxicating to be the center of someone’s universe and I think Marianne was particularly good at making Willoughby the center of hers. For example “When he was present, she had no eyes for any one else. Everything he did was right. Everything he said was clever” (Ch. 11) and of course Marianne “could never love by halves” (Ch. 50).

I think Willoughby is a calculating person but I never said everything he did was calculating. I hope you will agree there is a difference—a person can be calculating but still occasionally act spontaneously or without selfishly scheming towards a particular goal. I have not even had the time to focus on all his actions but I do think there is a case for the acts I have suggested are calculating. Of course you may see it otherwise but I was interested in why you do. I asked why you did not think he was calculating enough to avoid legal entanglements with women he wanted to enjoy, one way or another, but did not wish to commit to. The evidence in the text, his multiple past victims (Ch. 44) including Eliza and single status at the beginning of the novel, is he has been quite successful. I think Willoughby avoids legal entanglements on principle even if he believes the lady in question or her mother is unlikely to sue for breach of promise. I imagine any respectable rake and cad would do the same or their illicit careers would be short-lived.