I don't think that's true, prove it. Rather, even mut'ah is forbidden to be done with them if you are in a permanent marriage with a Muslim.
GreatChineseFall Here's a link to the rulings of Ayatollah Seyyed Ali al Sistani on the subject:
https://www.sistani.org/english/book/48/2348/No 2406 is relevant to your inquiry. Some other scholars rule that permanent marriage with Kitabi women is halal but makruh.
So mut'ah is used as a da'wah practice? Why is it forbidden for married men then? Or are only unmarried men suitable for this da'wah? Please, is it forbidden then to have children with them for as long as they are not converted? Or are you going to anticipate a conversion and produce children beforehand anyway?
Yes, naturally for Muslims with good akhlaq to spend time around non-Muslims can be a form of da'wah. Women converting to Islam to marry Muslim men, or due to being impressed by Muslims' adab, is a common occurrence.
In the case of children, to my understanding it will be similar to permanent marriage with Kitabi women whereby, assuming they don't revert, the situation is allowed but undesirable.
How generous of these guys to help these poor women out and how inconvenient for them that Allah has restricted a man to marry four women only. Can't he just help them out financially without a marriage or can't he guide them to other righteous and financially responsible men? Or is he the only suitable person around? In what world would this ever be the only practical solution to that problem?
Of course Allah(swt) has not made this Deen inconvenient for us. The above situation is one which I opined would be unlikely, though an eminent case would be that of a man away from home for extended periods of time.
For a man who isn't away from home, he may prefer to contract mut'ah than to provide support freely, or there may be other contingencies.
Finally, there it is, a partial admission after several posts. We are not there yet however, why don't you admit it fully while you are at it? To fulfill sexual needs doesn't just have to be an aspect of mut'ah, it can be the main purpose of it, can't it? So all those examples you just mentioned are just convenient examples. You have no problem with a man who lives with his four wives, and he isn't some place remote, and his wives aren't pregnant or unavailable or something, and his future mut'ah wife isn't financially in very difficult times or a widow, he simply doesn't have an "appetite" for his permanent wives, to just go and contract a mut'ah marriage with some woman or even multiple women for a few hours. You don't have a problem with this, as it's not only permissible, rather it is recommended and he is seen as someone who has done a virtuous act of worship. There lies the main difference with any other type of marriage.
An "admission" of something we're already aware of?
We already know that mut'ah is contracted for sexual relations to take place without sin; but it also has other social functions.
Regarding the "recommended" status of mut'ah, to my understating that's true but it depends on the circumstances. Marriage on the whole is recommended in Islam, and one of the many reasons for this is to satisfy physical needs in a halal manner. This is true for the Islamic view of marriage in general, so I don't concur with the "difference" you note in your last sentence.
It's highly unlikely that a man in the situation you describe above would look for a temporary marriage, especially since it's very rare for a man to have four wives to begin with. Therefore, this is more of a theoretical question than a real life one.
If in that situation, something was so amiss that his four wives weren't enough for him, it might be symptomatic of more deeply-rooted problems in the domestic sphere.
Temporary marriage may be halal for him, though that doesn't mean it would be the best thing for him to do.
Something I must remind you of is that according to all Muslims, the Holy Prophet(saws) made mut'ah halal and instructed the sahaaba to do it.
When you're questioning me from a skeptical angle, you're questioning something Allah(swt) and His Messenger(saws) made halal, according to your own beliefs.