In the Deuteronomy 23:4, it says that a Moabite cannot join the “community of G-d”—even to the 10th generation. Ruth was a Moabite, and was allowed to join. Her descendants, Kings David and Solomon were less than 10 generations from a Moabite (Ruth’s father).

I know the Mishna says that a male Moabite is not allowed to convert, but a female is allowed. Given that all commandments are expressed in the masculine form and apply to both genders (it says “lo tignov,” “Don’t steal” in the masculine form, yet it applies to females also, etc.), how are we to understand that the exclusion of Moabite converts is limited to men only?

That is an excellent question, and the Talmud itself asks a very similar question, albeit in slightly different wording.

As you mention in your question, the Scriptural source for the limitation of the restriction to men and not women is found in Deuteronomy 23:4: “An Ammonite and Moabite shall not enter the congregation of G-d…”. In Hebrew, the word for Moabite is Moavi, in the male form, which the Talmud deduces to mean only Moabite men. However, as you point out, much of the Torah is written using the male form, but nonetheless applies to both genders.

Nevertheless, as the Talmud explains, we cannot ignore the very next verse: “Because of the fact that they did not greet you with bread and water on the road when you were leaving Egypt, and because they hired Bilaam against you… to curse you”. This verse explains that the entire reason for the restriction on Moabite marriage is due to the fact that the Moabites did not provide the Jewish people with provisions when they left Egypt, and that they hired Bilaam to curse the Jews. Now, the Talmud continues, certainly, in ancient Moabite society, only the men could have made such a decision as whether to aid and assist the Jewish people or to hire Bilaam.  After all, the men were in absolute control of all financial decisions, and women had no say in financial matters! Therefore, only the Moabite men are held responsible for their failure to assist the Jewish people, and for the hiring of Bilaam.  Therefore, the Torah, by providing the reasoning behind the restriction on marriage, means to limit the restriction to males only.

In summary, the derivation of the limitation is not only from the usage of the male form of Moavi, but from the fact that the Torah additionally provides a rationale for the law, which singles out the males, and not the females, of the Moabite people.

Be well,
Rabbi Yoel Spotts