If a man is required to guard his eyes – how is it possible for him to be a witness at the bedeken.
Although a man must guard his eyes, there is no prohibition in seeing the face of a woman. The prohibition of “guarding one’s eyes” is on gazing for the sake of pleasure.
Note that according to the great majority of authorities, there is no obligation for the witnesses to see the face of the bride.
Mabit (1:226), and Maharit (Mabit’s son, in chiddushim to Kiddushin) write that witnesses must know the bride and see her face. However, the great majority of poskim dispute this, and rule that there is no need for the witnesses to see the face of the bride. See Avnei Miluim (31:4), Yeshuos Yaakov (Even ha-Ezer 42); Divrei Chaim (2:71); Beis Yitzchak (Even Ha-Ezer I, 97); and many others. Radvaz (IV, 156) writes that the custom was for brides to get married with their faces uncovered. Today, this is not the general custom.