Liu Xianglian married You Sanjie. Although Liu Xianglian's family is neither rich nor expensive, it is also the son of a noble family. Her family style is relatively clean, and she can't tolerate her wife being a defiled woman. This is from the family background and old customs.
One of the most important reasons is emotional problems.
When Jia Lian married Second Sister You as a concubine, Jia Lian had already taken a fancy to the beauty of Second Sister You, and Jia Lian had already become one with Second Sister You. Both people know each other and accept each other's shortcomings.
Liu Xianglian didn't know You Sanjie until Jia Lian introduced him. Liu Xianglian didn't know You Sanjie, but verbally gave Jia Lian a sword as a token of love.
Liu Xianglian learned from Jia Lian that You Sanjie was a stunning beauty. She didn't know what had happened to You Sanjie before, so she readily agreed.
But I didn't expect to meet Baoyu in Jiafu. Baoyu told Liu Xianglian about You Sanjie's past, which made Liu Xianglian disappointed in You Sanjie and felt that he was kept in the dark by Jia Lian. I couldn't accept this fact for a while, so I asked your third sister to break off her marriage.
In fact, Liu Xianglian is a good man, and there is no affection between him and You Sanjie. At that time, it was understandable that Liu Xianglian could not accept You Sanjie. If he and You Sanjie have feelings, and then decide whether to get married, I think the result will be different.
You Sanjie may not be his first wife, but I think Liu Xianglian can accept her as a concubine.