The antonyms for "becoming tender" would be "becoming tough" or "becoming hard." When something becomes tough, it means it has lost its softness and pliability, which is the exact opposite of becoming tender. Similarly, hardness also implies the absence of softness and flexibility, which hinders the ability to become tender. These antonyms are commonly used in cooking, where the quality of tender meat is desirable, but tough or hard meat is not. In the context of relationships, the antonyms of becoming tender could be becoming indifferent, cold, or aloof, which are not desirable traits for healthy relationships.