Only because on the battlefield of the two armies, Lin Chongsheng captured the beautiful female general Hu Sanniang alive, which led to the gossip and gossip between the two of them on Liangshan.

The source of the scandal;
When Liangshan Po attacked Zhujiazhuang, The Third Lady came to the battlefield of the two armies as a foreign aid to Zhujiazhuang and openly challenged the good men of Liangshan. The short-legged tiger King Ying, who came with the army, saw that the third lady was young and beautiful, and immediately became interested, so he urged his horse to come to the battlefield and wanted to capture the third lady alive, but he did not think about it but was captured by the third lady. After that, Ou Peng and Ma Lin of the Liangshan camp also came to fight against the Third Lady one after another, and as a result, they all lost the battle and left.
After winning three consecutive battles, and seeing that Song Jiang was alone, he flew after Song Jiang, and did not think about it halfway but encountered Lin Chong who came to save Song Jiang, so the two fought together, however, within ten rounds, Hu Sanniang was captured alive by Lin Chongsheng.
Hu Sanniang learned martial arts since childhood, a pair of sun and moon double knives in the palm of her hand, it can be said that it is rare to meet the enemy, in this battle, she has won three consecutive battles in Liangshan, it can be said that morale is high, however, the appearance of Lin Chong directly captured her, that is to say, at this time, Hu Sanniang, who has a girl's heart, was conquered by Lin Chong, and it is said that beautiful women love heroes, in this case, Hu Sanniang will naturally be moved by Lin Chong, the big hero.
If after that, If Third Lady Hu and Lin Chong could come together, naturally there would be no gossip, and I didn't think that after Zhujiazhuang was broken by Liangshan, Song Jiang married Third Lady Hu to Wang Ying, who was short, ugly, and incompetent, just imagine, how could Third Lady Hu willingly accept Wang Ying? So the gossip about the relationship between Hu Sanniang and Lin Chong spread in Liangshan.
As for the statement that before the last battle in Liangshan, the statement that The Third Lady visited Lin Chong at night was not mentioned in the original text of the Water Margin. This incident was just something that posterity imagined based on the scandal between Hu Sanniang and Lin Chong. I want to leave no regrets between Hu Sanniang and Lin Chong.
So, can this happen?
Obviously, this is something that cannot happen. The rationale for this is as follows:
After being captured in Liangshan, The Third Lady recognized Song Jiang's father Song Taigong as her righteous father, and after that, Song Jiang made the lord to marry the Third Lady to Wang Ying, that is to say, the marriage between the Third Lady and Wang Ying was a formal marriage with the words of the parents' fate, "marrying and obeying the husband" was a rule that ancient women had to abide by, and the Third Lady, as a bridesmaid, naturally had to abide by this rule, even if she was more dissatisfied with her husband, she would not do the red apricot out of the wall, and "wear a green hat" for her husband!
There is another reason, that is, since Hu Sanniang has already recognized Song Taigong as her righteous father, that is to say, the supreme leader of Liangshan, Song Jiang, is her righteous brother, in this case, Hu Sanniang will certainly not do anything that damages Song Jiang's image, especially the most taboo "red apricot out of the wall" that the ancients are most taboo, so the saying that Hu Sanniang visited Lin Chong at night does not exist at all.