In large part, the answer to this question depends on whether you believe Hosea 3:1–5 is a retelling of the same event which took place in Hosea 1:1–11 or an account of a different event some time afterward. If we go with the former, we must refer to our commentary on Hosea 1:2, namely that Gomer was a prostitute. But since we have already stated in the prior question and answer that “again” implies a sequence of events rather than a retelling of the same event, it is more plausible that Gomer’s sexual sin is, in fact, adultery against Hosea, her husband.1 If it was shocking enough for Hosea to marry a known prostitute, it is equally shocking for Hosea to here be asked to return to Gomer, now that she has violated her marriage covenant. Nevertheless, we must remember that this serves to underscore the horror of Israel’s spiritual adultery with their covenant God. Thus, the choice between prostitution and adultery will not fundamentally change Gomer’s symbolic role. Either way she represents the unfaithful Israelites before a faithful God.
1 L'Eternel me dit: Va encore, et aime une femme aimée d'un amant, et adultère; aime-la comme l'Eternel aime les enfants d'Israël, qui se tournent vers d'autres dieux et qui aiment les gâteaux de raisins.