I'm genuinely curious, would the OP be vile and abusive and the husband be an abused victim if she'd posted that she caught him cheating and called him names then?
Can you imagine that? "Have just caught my husband shagging the neighbour in our bathroom, before we married we agreed that we would have a monogamous relationship where neither of us would have sex with anyone else and he said that it was definitely what he wanted and that he agreed with my views on monogamy completely but apparently the neighbour has been shagging all his mates and offered him a turn too so he took it. He's a piece of shit and I've lost all respect for him"
Cue endless replies of:
'You're so controlling, he's an adult and if he wants to shag his neighbour that's up to him'
'Poor bloke should get a lock on the bathroom door so he can shag neighbours in peace'
'You controlling psycho, personally I encourage my husband to sleep with everyone on the street'
'I can't believe you swore on the internet about him, that's just as bad as sleeping with someone behind your back and against your agreement, worse probably'
'He only agreed not to shag other people because you made him, clearly he was lying and so just get on with it'
'Omg it's not like he went round to his neighbours house and asked for sex, she came to him, he probably only did it out of curiosity'
'Why are you being mean about his friends, it's none of your business if they sleep with the neighbour behind their partners backs and encourage your husband to do the same'
'Totally unrealistic to expect a red blooded male not to shag the neighbour'
'If a woman posted that her husband wouldn't let her sleep with the neighbour or anyone else for that matter we'd tell her to LTB'
'I can't believe you want him to be ASHAMED of shagging the neighbour you bullying abusive harridan'
Etc etc etc
She didn't catch him with his penis in another woman, but she did catch him doing something that was explicitly against the promises they made to each other and the boundaries of their relationship. No, venting and being upset and hurt on an anonymous internet is not the same as that, it's just not - even if she used some swear words.
It might not be your boundary, it's not mine for the record but that's not the point. If you've agreed no cheating and your partner cheats then you get to be angry/hurt/furious and to vent with as many swear words you can think of in my opinion, maybe even invent new ones for the purpose. Which is just the same as if you agree to no porn and your partner breaks that MUTUALLY AGREED AND MUTUALLY 'ENFORCED' (for want of a better word i.e. Something they both expected and required of the other) BOUNDARY. Particularly if he does it in such a way as could expose the children to it, and also allows his mates to make her into a laughing stock into the bargain.
Wouldn't matter what the boundary - if you both agree, freely agree, that your relationship will not contain cheating/porn/violence/drug use/ gambling/ Scientology/ hamsters etc etc etc then it is not controlling to expect the other person to stick to that which they too said was what they wanted. If they can no longer stick to that and feel urgently compelled to start having sex with strangers/ watching the disturbing porn their friends send them / get a hamster then they can talk to you and see if you can renegotiate boundaries, and if not then either do without or leave.