Was Nehemiah against marrying Moabitess?

The Canaanites were descendants of Canaan (Gen 10:6). The Moabites were descendants of Moab (Gen 19:37), a grandson of Lot. They occupied different areas, but they were neighbors.

Moses commanded the Israelites not to intermarry with Canaanites Deuteronomy 7:

1 When the LORD your God brings you into the land that you are entering to possess, and He drives out before you many nations—the Hittites, Girgashites, Amorites, Canaanites, Perizzites, Hivites, and Jebusites, seven nations larger and stronger than you— 2 and when the LORD your God has delivered them over to you to defeat them, then you must devote them to complete destruction. Make no treaty with them and show them no mercy.
3 Do not intermarry with them Do not give your daughters to their sons or take their daughters for your sons,

Click to expand...

Do not intermarry with Canaanites. Why?

4 because they will turn your sons away from following Me to serve other gods. Then the anger of the LORD will burn against you, and He will swiftly destroy you.

Click to expand...

It was okay to take a Moabite woman. Boaz married Ruth (4:13).

Deuteronomy 23:

3 No Ammonite or Moabite may enter the assembly of the LORD. Even to the tenth generation, none of them may enter the assembly of the LORD forever.

Click to expand...

They were excluded from the formal assembly of the LORD. However, converted foreigners were allowed to participate in other celebrations of the LORD.

After the Babylonian exile, Nehemiah 13:

1 On that day they read from the Book of Moses in the hearing of the people. And in it was found written that no Ammonite or Moabite should ever enter the assembly of God.

Click to expand...

Nehemiah found this command of Moses. He continued:

23 In those days also I saw the Jews who had married women of Ashdod, Ammon, and Moab.

Click to expand...

Ashdodites were Canaanites; Moabites were not.

24 And half of their children spoke the language of Ashdod, and they could not speak the language of Judah, but only the language of each people. 25 And I confronted them and cursed them and beat some of them and pulled out their hair. And I made them take an oath in the name of God, saying, “You shall not give your daughters to their sons, or take their daughters for your sons or for yourselves.

Click to expand...

I.e., regarding the Ashdodites/Canaanites.

26 Did not Solomon king of Israel sin on account of such women? Among the many nations there was no king like him, and he was beloved by his God, and God made him king over all Israel. Nevertheless, foreign women made even him to sin. 27 Shall we then listen to you and do all this great evil and act treacherously against our God by marrying foreign women?”

Click to expand...

There was no law against marrying foreign women universally. However, according to Deut 7:4, marrying an idol worshiper was a bad idea.

Was Nehemiah against Israelites marrying Moabitess?

Not exactly. He lumped the Moabites with the Canaanites because they were all pagan groups. More to his point was that He was against Israelites marrying a pagan.

Today, we can apply Nehemiah's wisdom for Christians not to marry non-Christians.