Wait wait wait, hang on. Is the man some sort of prostitute?
He comes over for sex regularly, they are not dating or boyfriend/girlfriend, he is there for her pleasures, etc. So is he a prostitute?! Is that why you have a problem with the situation?
Because I think that could possibly change attitudes. I'm not certain that I'd be terribly cool with an actual prostitute in the house with my kids...
I do wonder how/why you are so certain that they are not dating. Having regular sex is a pretty good indicator that people are indeed dating.
I think the reason that you believe we are all "stupid" "trolls" is that nobody here sees immorality in pre-marital sex between two consenting adults anymore. People just don't find that disagreeable in this day and age. As long as they are not being overt about it, are not screaming and banging on the walls, not keeping everyone awake all night, then the vast majority of people here (and in the rest of the Western world) see nothing at all wrong with having sex before getting married.
We are not "stupid" or "trolls", we are not taking the piss, we are not trying to be contrary, and we are not even missing the point you're trying to make. We simply completely disagree with the reasons that you are upset and truly believe that you are the one being unreasonable. That's all it is. By telling you to leave it off because it is not your business, we are trying to HELP YOU. Genuinely!
Since you seem to be searching for other advice, though, the best I can offer - that is, if the man is not actually a prostitute! - is this:
If you need to tell your children something because they are asking questions or you're concerned that they will, and you are not comfortable with opening your children's eyes to pre-marital sex happening, then just tell them that the pair are already married, and that's why they sleep in the same room. Outside of leaving the house, which you said you cannot afford to do, I don't think you've got any other options.