At least Stannis had already raised and trained Shireen as his heir so she knew her duty both as a woman and as a ruler. Also, she was a daughter of a King. A short-lived King sure, but one nonetheless. The blood of the mighty Storm Kings flew thickly through her veins. Shireen might not be an exceptional beauty, but once the greyscale was gone, she was not ugly by any standard and was even blossoming more and more with every single day. Not to mention that Shireen was kind, sweet, and shy. Taking her as a wife would make for a peaceful marriage.