How can one deal with a Canadian Christian classmate who insults the Prophet, peace and blessings be upon him, claiming that he was a womanizer and married a child, and that his marriage to her was rape, and that he invaded countries and forced people to convert to Islam by the sword?

The Prophet's love for women is a natural inclination and not a defect. Allah Almighty mentioned the love of desires, including women, as a natural matter. The Prophet's marriage to numerous women was for social and Da'wah (preaching) purposes, and to convey religious matters that might be hidden from men, and not for the pursuit of desires. As for his marriage to Aisha, may Allah be pleased with her, it was with her father's permission, and he consummated the marriage with her when she was at a suitable age for that time. She herself used to be proud of this marriage, which refutes the شبهة (insinuation/suspicion) of rape.