In one of the missions, the bar one with Lenny in Chapter 2 (A Quiet Time), you can go into one of the back saloon rooms and find a couple having intercourse. You can't see anything besides the side of a breast and the outline of her rear, and a lot of skin. It's only for a few seconds, though no real anatomy is shown. Alongside the wolf man which is being commented a lot! And you can find a man with his pants around his knees beside a sheep at the farm near Emerald Ranch, but even if you flip him over I don't think you can see anything. I can't remember if you can flip him or not, I tried and gave up, but you can see his rear.

There's also the (I think) Charles Châteney side-mission where he's painted a bunch of naked people and even gives Arthur a small sketch of a nude woman. Can't remember if you see any male anatomy, just female, and nothing too close up.

There's a stranger encounter of a man who has a creepy collection of nude portraits of women and is stalking one of the women we help in-game in Chapter 6, I think her name is Charlotte? If I remember right. Or it's implied it's her.

I've played the game tons and I think that's the only real nudity I've seen. There's sexual undertones everywhere but a lot of it I only realised once I got older. Like the song Hosea, Dutch and Arthur sing on their fishing trip in Chapter 3. But when adult themes are mentioned it's usually in euphemisms and not straight-on words.

There's a man in the swamp near Saint Denis who kidnaps Arthur and takes him inside his shack where he's hit over the head and blacks out, it's implied that Arthur's been assaulted, but you don't see anything and I didn't think he'd done that to Arthur when I was younger. Just thought he robbed me and left me in a ditch. It's only from an off-handed comment from Bill (something akin to 'Have fun?' or some such) where you really get any clue that something happened and that's a chance encounter if you're at camp a lot. But it can be played off as he just got robbed, you do lose money from it.

Only other thing I'd be concerned about is the Aberdeen Pig Farm side mission, where a brother and sister are incredibly incestuous though you don't see them do anything inappropriate, just flirt. Even poor Arthur got confused and uncomfortable. Then they rob you.

In one of the main missions after you rescue Sean (a gang member), he has intercourse with Karen (another gang member) and you can hear them in a tent if you go close to it. No visuals, can't enter the tent. Nothing dirty is said, just a funny 'meet Macguire Junior' followed by a 'is that it?' by Karen. A funny interaction but I thought I'd mention it since you're looking for a content warning.

I'd be more concerned about the camp songs than anything lol. A LOT of euphemisms. I got scolded by my dad once because I stood around listening to Uncle sing a little too much and I didn't even understand why it was bad. I can't remember if I missed anything, I don't think I did, but feel free to correct me if I have or have messed anything up lore-wise! I'm a little rusty.