I see, so it seems perhaps some misunderstanding happened.

My suggestion to you, and to Miss Relena, is to let some time pass. Zechs loves his sister, and only wishes to see her happy. Considering he has had little contact with her even before this point, it would...serve to reason that he would respond so strongly when he sees her becoming involved, and particularly with someone who he has, in his mind, clashed with recently.