This would literally turn Hotwife Phone (the current game) into Cuckold Phone (your fantasy), ie it would be kind of a bait and switch that would destroy all the groundwork laid out until now. In fact, Le Stag specifically went into the opposite direction in update 0.21, when he gave Nathan a girlfriend (who is specifically *not* Laura). It's hard to see how he could be clearer about that (but at this stage, I guess even Nathan marrying Sophia and MC marrying Laura probably wouldn't be enough to convince those who desperately crave for the ultimate-cuck-go-scorched-earth-on-the-current-relationship-please route that they won't get what they want).
This is just not that kind of game, guys. Games where MC is cucked repeatedly by his wife are a dime a dozen, so you probably won't have any trouble finding what you're looking for elsewhere. Also, Sophia gives Le Stag an "out" for Nathan, who can now disappear from the game at any time, either temporarily or definitively, should the story warrant it.
As for Ashley's and Tara's reactions, it's maybe not that hard to imagine them. Tara first. MC is basically the only guy she trusts at this point, and I don't think she would enjoy seeing him "destroyed" in this way (she needs an active MC to interact with and help her out of her shell, not a passive hollow shell of a man). There's a good chance she would lose all respect, if not for MC, at least for *Laura*, for submitting her boyfriend to such an indignity. And she would probably exit the story never to return at this point, imho.
Ashley is more of a wildcard, granted. She *loves* teasing MC, calling him "cucky" etc., but notice she never goes too far. In fact, update 0.22 is
very interesting regarding Ashley (and Laura). That's a spoiler if you haven't played it yet, but it almost brings the full madhouse+ (including Nathan and Marcel) relationship in a new light, especially as regards to the Ashley-Marcel dynamics. Update 0.22 is basically Laura and Ashley playing with their respective toys while waiting for the man of the house (MC) to get back to his harem (Tara's word, not mine), ie the complete oppostite of what you're asking for.
Nathan almost got nothing out of Laura twice--he ended up creampying her for the first time, but she found denying him so hot that I wouldn't be surprised if she started ruining him soon (which would flip the dynamic that MC/Laura/Nathan had at the start on its head, ie it's *MC* who is steadily gaining ground in Laura's heart and taking a more prominent role in the relationship, not Nathan). As for Ashley, she obviously totally has Marcel under her spell and now seems to want MC as a bull (it might be just teasing, sure, both for Laura and MC, but after she denied Marcel and had him jerk off to her before servicing her with his mouth, Ashley was ready and willing for MC to come fuck her while Marcel was still in the room. Would she have gone with it if Laura had said yes? Pretty sure she would have--and unless Laura needed Marcel to entertain her, there's a good chance that Ashley would have directed her BF to stay in the room, watch and jerk off again while her new bull fucked her--which Marcel hadn't been allowed to do all evening).
Ergo, neither Ashley nor Tara want a weak MC (the game is heading in the opposite direction and will almost certainly give you the opportunity to get all the girls before the end should you want that), and I don't think they would be turned on by that. Tara would probably hate it, and even if Ashley may find it fun at first, she likes that MC is sometimes able to resist her (up to a point, ofc

), or at least hold his own against her. Noone in the madhouse would "benefit" from the MC being subversient to Nathan, imho (Nathan wouldn't either, he's not Oliver or Mike; and for Laura, well, I have a hard time seeing how the Laura that we've known since the beginning of the game would enjoy such a development--but hey, maybe that's just me).