I am under guardianship and wish to get married. My guardian says he will have a marriage contract drawn up by a notary, even though I do not want one. Can he do that?

Yes. Since the law of 23 March 2019, a protected adult (under guardianship, curatorship, etc.) no longer needs to obtain a judge’s authorization to marry. Only the curator or guardian must be informed of the intended marriage (Art. 460 Civil Code).
However, the guardian may request the judge’s authorization to have a notary draw up a marriage contract if he can demonstrate that it will protect the interests of the protected adult (Art. 1399 Civil Code).