Thanks all for your responses, both the jokes and the more serious discussion. I appreciate both.
Here are my clarifications, conclusions, and random thoughts.
1. I didn't get to the video via YouTube. I got to the video from the Home Page of DnD Beyond.
It is a service I've paid a lot of money for. I let my son set up an account an have access to my purchased content. He prefers using DnD Beyond to build his characters and has even gotten into making his own homebrew monsters on DnD Beyond. Until that video, I never thought much about monitoring the content on that site. To be clear, my reaction was not one of great offense, get the pitchforks! No, it was simply, huh--maybe I need to monitor this site more.
2. I liked the video
Personally, I enjoyed the video and thought the "weird orgy" remark was funny. It just seemed out of place for a video highlighted on their landing page.
3. For a 13+ site--not a big deal, but is something that does call for some parental scrutiny
I realize it was an off-hand, non-explicit joke. But I think that I am well within the mainstream when it comes to my knee-jerk reaction. Again, my knee-jerk wasn't "orgies are evil, burn the site to the ground." It was simply, hmm, this may not be a family friendly site and I need to think about whether I should monitor it more if I'm going to let my kid use it.
I guarantee that if I make that same joke to another parent while in front of someone else's kids, there is a good chance I'll be called out on it.
4. I agree that you can explain these things to pre-teens
By age 13, at least in my part of the United States, kids have already had some sex education. My kids have friends with gay parents and they have a gay aunt. They understand what homosexuality is and see it as not common, but not strange, weird, or wrong. My kids are non-circumcised in a part of the United States where the majority of boys still are--so we've discussed that. I have a transgender player in one of the games I run at home. So my wife and I have had that discussion with the kids.
I have not had either son ask me what an orgy is yet, but if one did, I would simply explain to him that it is when a group of consenting adults have sex together. I'm pretty sure he would say ick, roll his eyes, and laugh at the idea and then move on to talk about something that actually interests him. So, yes, I agree. You can talk to kids about these things. BUT I WOULD ALSO ASK "where did you hear about orgies?" I would expect it to be something some friend said or something he heard older kids joke about at school. I would not expect it to be from a video interview with a WotC game designer on Why DnD is so Memorable on the home page of DnD Beyond. It would make me think about whether it is a site I want him to have access to.
5. There are a lot of mature themes in DnD, even in core books -- so what should a parent's expectations be?
Is a joke about "weird orgies" that surprising when the Monster Manual describes Succubi and Incubi "lurk[ing] next to a mortal’s bedside and whisper[ing] forbidden pleasures. Sleeping victims are tempted to give in to their darkest desires, indulge in taboos, and feed forbidden appetites. As the fiend fills the victim’s dreams with debauched images, the victim becomes more susceptible to temptation in everyday life." I let my kids read the monster manual, so why should I have an issue with the video. I guess it was just not expected and it is also more explicit as it used a work that references a specific a sex activity rather than skirting around the topic.
Parents do need to understand that D&D is primarily targeted at older teens and adults. So, I do think it is fair for people to tell me that any surprise at this one video is misplaced if you know anything about D&D, both official and fan content.
6. Yes, I do understand I'm a hypocrite
It is hard to be 100% consistent in your parenting. I don't want to ban video games, but I want to control screen time. I don't want them to use curse words, but I'm not particularly concerned with them hearing it. You don't want to prohibit the use of social media, but you know the dangers and pitfalls and need some level of monitoring and limits. And you are going to have rules in your home that their friend's parents don't have in theirs.
Most thoughtful parents will have to constantly measure their reactions. You have to. The measures keep changing. And your kids are changing and growing older. There are language and jokes in movies that my son has seen that are far more explicit than anything in official DnD material. But I know it is there, I expect it, I'm used to it, and the industry at least has a rating system to help make some informed parenting decisions. When you don't expect it--it is going to make you scrutinize the source more than you would other content.
If you are not careful, however, it can lead to a situation that many comics are familiar with. Selective outrage. I can laugh along as you joke about all manner of topics until you hit a sore spot. There are lots of things in the game that I know will bother other parents that don't bother me. I don't have an issue with language in the Players Handbook that discuss not needing to be confined to binary notions of sex and gender. I don't, for the most part, have a problem the fighting, violence, and killing in the game. The demons and devils don't bother me, because I don't believe they are any more real than beholders and illithids. So it does seem to be strangely selective to be questioning a throw-away orgy joke.
I'm not going to remove my kid's access to DnD Beyond because of this. Actually, nothing will change, other than I'll pay more attention to the content published on DnD Beyond.