"My boyfriend is redheaded."
Translation:Mon copain est roux.
It could be "mon ami" for a boyfriend, and yes it's definitely a very serious boyfriend. Whereas "mon copain" is just someone you're dating. Not very serious at all. I believe "copain" came from the word "pain" (bread), basically to describe someone you share or break bread with. (Anyone feel free to correct me if I'm wrong).
In my experience in France putting "ma/mon" in front of any of these words makes them possessive, and I would say the rule is if it's possessive, it's a romantic partner in some shape or fashion.
"Mon ami(e)" CAN also mean like a very close or best friend too. But unless the person you're talking to knows both you and this person and knows that you aren't dating, then they are likely going to assume that you mean it's a boyfriend/girlfriend. You can say "un(e) ami(e)" to clarify it's platonic if you want.
*These are the cultural meanings I've found behind these terms while living in France (Paris and Lyon).
If you lived in France, you should have noticed that:
"Ma petite amie/Mon petit ami" and "Ma petite copine/Mon petit copain" means that they are girlfriends/boyfriends.
It is the use of "petite/petit" that makes it "girlfriend/Boyfriend"
Using the term "Copine/copain" which means "friend", is more used between young people since it is pretty informal.
Mon amie/Mon ami also meaning "friends" is more formal and I would even say more elegant, more polite and more respectful, regardless of the ages of the people involved. In France, one cannot get it wrong when using "Amie/Ami".
In addition, in France, the meaning of friendship is stronger and more true when one uses Amie/Ami. When you are introduced as a "copain" instead of "un ami", you know where you stand!..
In effect, "copains" is like using "mates, pals, buddy, dude etc." It is not wrong but it is more common.
"My boyfriend is redheaded." DL's translation: "Mon copain est roux" is wrong and should be : "Mon petit copain est roux" AND/OR "Mon petit ami est roux"
These are the literal translations, like Sitesurf already posted.
But no, Duo's translation isn't wrong. It seems like modern young french people are changing the meaning of these terms to all mean a romantic partner (but not a spouse). I suppose that 20 or 30 years ago they probably did only mean "friend". It's a whole lot more confusing now.