SHORT ANSWER: While I can't say it is "wrong", I do not see a happy future for two Hmong youth with the same last name who date or get married. Therefore, I don't think it's a good idea and would strongly discourage it in all circumstances.
LONG ANSWER: This is a pretty hot topic right now among Hmong young adults. I wouldn't be surprised if you've even had a few conversations about it. Some people say absolutely never, others just get a squeamish look on their face, while others take a "love conquers all" approach. But this is actually a very complex issue with lots of cultural and social elements to it.
Understanding the Cultural Taboo
If you're Hmong, then I'm sure you know what your parents would say about dating someone with the same last name: "Ua tsi tau!!!" (That can't be done!!!) But have you ever wondered why there is such a strong cultural prohibition against it? There are a lot of other cultural no-no's (like marrying a mekas guy) that have been broken (lucky for me). So, why not this one?
Here's my theory on it. As you know, Hmong culture says that a person with the same last name is kinda like a brother or sister--and should be treated as one. So, marrying them would kinda be like marrying a brother or sister. (Ick!) Anthropologically speaking, every culture across the globe has very strong prohibitions against incest, so it makes sense that the Hmong would be no exception. If everyone with the same last name is viewed as being in the same family, then two people from the same family dating would be considered incest by the Hmong community. When I started to think of it in these terms, the power of this taboo and people's strong reaction against it started to make more sense.
What Does It Say in the Bible? Don't We Have Freedom in Christ (To Date Whomever We Want)?
First, let's be clear about something. The Bible doesn't explicitly forbid this type of marriage, i.e. between two people from the same clan. (In fact, there were plenty of marriages in the Old Testament that were very…um… 'close'.) So, instead, we have to look for more general principles that can help us understand the issue.
For example, I've heard some arguments from single adults that it would be okay to marry someone with the same last name because we have freedom in Christ. While it is absolutely true that have freedom in Christ (from religious laws, condemnation, death, etc.), the question still remains "How should we use that freedom?" Christian liberty isn't license to do anything we want, as Paul clearly presents in 1 Corinthians 10:23-33.
23 "I have the right to do anything," you say—but not everything is beneficial. "I have the right to do anything"—but not everything is constructive. 24 No one should seek their own good, but the good of others.
25 Eat anything sold in the meat market without raising questions of conscience, 26 for, "The earth is the Lord's, and everything in it."
27 If an unbeliever invites you to a meal and you want to go, eat whatever is put before you without raising questions of conscience.28 But if someone says to you, "This has been offered in sacrifice," then do not eat it, both for the sake of the one who told you and for the sake of conscience. 29 I am referring to the other person's conscience, not yours. For why is my freedom being judged by another's conscience? 30 If I take part in the meal with thankfulness, why am I denounced because of something I thank God for?
31 So whether you eat or drink or whatever you do, do it all for the glory of God. 32 Do not cause anyone to stumble, whether Jews, Greeks or the church of God— 33 even as I try to please everyone in every way. For I am not seeking my own good but the good of many, so that they may be saved.
Here's a summary of this passage: We have freedom in Christ to do anything (that isn't sinful), but not everything is good for us to do. So, we should not insist on our freedom but lay it down for the sake of others.
So who are those "others" that you might need to consider? Let's look at two different groups of people who would be impacted: the parents and the larger Hmong community.
Impact on the Parents
Another topic to consider is the impact such a decision would have on the couple's parents? First, it would bring an incredible about of shame, criticism, and loss of face onto the parents, grandparents and relatives of the two people. Because of the strong collective nature of Hmong culture, when someone chooses to break the norms, people can be relentless in their criticisms and even sometimes cruel. The parents would never hear the end of it from practically every one of their relatives and friends. It might even be enough to permanently and irreparably damage the relationships between the children and their parents because the parents just absolutely cannot accept what they consider to be an unforgivable act.
Secondly is the issue of the biblical model for a child's relationship with their parents. While everyone is commanded to honor their parents (Exodus 20:12), children are also commanded to obey them as well (Ephesians 6:1). So, honoring parents is a life-long command, but obeying is limited by age or stage in life. Now, there's a lot of debate about exactly who is a "child". My simple take on it is that if you're still living at home and dependent on your parents, then you still fall under "child". So, if that describes the couple because each is still living at home with their parents, then there's the issue of obedience to consider.
Impact on the Community
The other group of people that would be impacted by this decision is the larger community, such as churches, schools, and social circles. I don't think it's an exaggeration to say that most of the Hmong community would react negatively to a dating or married couple with the same last name. At best, they would exclude the couple from any involvement; at worst, they would reject and shun them completely. This would probably even include the Hmong churches. The taboo is just too big for most Hmong people to ignore, look past, or change their mind about.
I would even predict awkwardness and uncomfortable feelings even around the younger generation. I don't think this taboo will change any time soon, so this couple would need to prepare themselves for 10, 15, even 20 years of personal criticism, social rejection, and possibly even anger and hatred directed at the couple and most likely their children as well. It might even mean that you leave the Hmong community entirely, just in order to have a peaceful, "normal" relationship and family life.
So, this is why I don't see a happy future for any couple of the same last name who date or marry. The taboo is just too strong and deeply embedded, and the reaction by family and the community would be just too devastating. So, this is one example where love doesn't conquer all, or if it does, it pays a pretty high price.