Mario was truthfully an introvert around females. Any male who had been trampled by Sharmie that often would more or less develop masochistic tendencies. Furthermore, he had very little experience in the aspect of love. Other than training, there was very little free time left in his life, which was either spent being monkeyed around by Sharmie, or playing babysitter for the squadron. Although he did not have a domineering personality, and had very good relationships with other people, he wasn't someone that girls would like. Therefore, he basically had zero experience in dealing with girls. Coupled with the fact that the girl before him was really cute, very beautiful, had put up a pitiful look, and with so many eyes staring at him from behind, he really did not know exactly what was the right decision to make here.
As for the matter about Sharmie being a girl, Mario had immediately tossed that thought aside. His big sis was absolutely not feminine at all!