That statement is inaccurate.
The thing is, Ghoul's in this series don't have the concept of marriage. So, why would they wish or not wish to marry someone when such concept didn't even exists for them.
As for the reason why Ghoul's didn't have that concept, its primarily because of their low fertility rate. They couldn't just stay fixed with one partner when the chance of successful conception was so low to being with. So, they simply created the open community lifestyle, were no one is in a permanent relationship, and any child born was cared and nurtured by the whole community.
But since Van had joined their group and fixed their fertility issues, plus inclusion of other Races (both living & Undead) which had the concept of Marriage, the Ghoul's had learned the concept and have decided to slowly accept it.
As for all of Van's harem members, all of the Ghoul Women actually interested in him have shown desire to marry him, once he becomes an adult and capable of reproducing.
Telea, being a former Human, already plans to marry Van.
So is Basdia, who had decided to marry Van now (previously she only wanted to bare his child, and eventually marry her daughter Jadal).
