In Kedushin on 30b - regarding the mitzvah to marry ones son off

The gemarah says that "It makes sense that he could marry off his son but how could he marry off his daughter?" But my question is why does it say you could marry off his son, a father is not allowed to marry his son off?

I think the gemara and Rashi follow the idea that the man is the active party in a kiddushin, and the woman is the (somewhat) passive accepter of the kiddushin. The action comes from the man. So a father is able to marry off his son (and the gemara 29b says that the son will generally accept his direction till age twenty or so), but as for his daughter, he needs to wait till he receives an offer. The gemara answers that he can do things to make her attractive, so that the offers will come.