It could be an attempted joke or it could have been serious, probably about 50 50 chance of either. Don't see how a bunch of strangers on the internet can judge it. Some context of the prior conversation might be useful - if for example someone had made a joke about taking the stag to a gay bar, then could easily see his comment as a witty follow up. If they'd been discussing going karting or clay pigeon shooting, then obviously not. Or if the prior conversation was full of lewd remarks by the others then his wouldn't seem that odd.
The fact the OP says partner has adhd, says things impulsively, and that they're all tradies, suggest he could just be trying too hard to fit in, misjudged the tone, and it was an attempt at a joke that failed miserably. The virtual tumbleweed response could indicate that rest of the chat group didn't find it funny and chose to ignore the immature behaviour, or that they felt somewhat rumbled by him saying what they were all thinking - i.e saying the unspoken rule. However, I'd have thought a bunch of crude tradies would have found it funny if it were intended as a joke.
I've got some friends who are tradesmen, and I've seen their group chats. The stuff they share and their 'banter' is absolutely vile. However, I know them well enough to know that they are 100% loyal to their partners. These conversations are just 'old fashioned' workplace crudeness, where they're all trying to fit in, shock and outdo each other. Thankfully it's dying out, but not yet extinct.
I've never been on a stag do where there has been any infidelity, nor any party wide suggestion of it. It certainly isn't that all stag parties are married men shagging prostitutes. I think that's a tiny minority, and generally the blokes that will do that on a stag do are probably doing it the rest of the time too. There is a small chance that if some of the men in the group are up for that type of behaviour then they'll manage to cajole others in the group to partake too. But any decent person will have the willpower and self control to behave themselves.
By the same measure, there was a thread on here last week where a woman got carried away on a hen do and gave a stripper a blowjob, does that mean that all women on hen dos are cheating on their partners?
Incidentally I would think that brothels/sex workers would provide condoms, so if that's where he was suggesting then it sounds like he has no experience of them, or that it genuinely was intended as a joke.
The OP's partner's response to being challenged would be telling too. A decent bloke would be mortified at his partner finding message, many would probably pull out of the stag do to try and salvage their relationship. A bloke who can't see the issue in what he's said would be more concerning.