You are referring to this Hadith, which is quoted in Musnad Ahmad, Musnad abi Ya'la and al-Kabeer of Imam at-Tabarni etc. the major problem here is the main route and known narrator of the sirah ibn Ishaq who is considred as not reliable in hadith matters, even the son of Imam Ahmad said quoting his father: لم يكن يحتج به في السنن No body used his narration when it commes to sunnah/hadith as an argument.
So scholars tend to say the hadith is da'if because hadith of ibn Ishaq isn't reliable if there's no similar narration of any other on the matter, which is the case here all the scholars who compiled this hadith have ibn Ishaq in the narrator chain.
An other narrator who is not reliable in the chain is Hussayn ibn Abdullah ibn Obaydallah ibn 'Abas حسين بن عبد الله بن عبيد الله بن العباس : he was a known narrator of manakeer.
Two major scholars have qualified this hadith as dai'f: Sheikh Shoayb al-Arnaout in his examination of Musnad Ahmad and Sheikh Hussayn Salem Assad in his examination of Musnad abi Ya'la. (See also this thread in Arabic on the matter)
An other note on the hadith matn, assuming it would be accepted: The Messenger of Allah (peace be upon him) only said that he would marry her if.... if she reached the right age for marriage, so this doesn't have nothing to do with desire, but could be related to strengthen the relationship to al-Abbas as he did with Abu Bakr and Omar (May Allah be pleased with them). So this hadith shows the position of al-'Abbas and his ranking by the Messenger of Allah.
And Allah knows best.