I have a long theory about the plot twist.I think the master mind behind the marriage is the prosecutor groom.I know it is wild ,but if you put him into equation ,all the situations are suddenly possible.
1) The Phone call that FL got while drying ML's hair--
She yelled at the caller that she wouldn't back down from 'it' like it is some 'deal' and she confirmed that the caller should keep the 'promise'. What's this 'promise' about?
And ,as we know FL was unaware that her family and step father il doh ,all knew about her relationship with ML.So the only possible figures with whom she can talk about her relationship are Prosecutor yeom and his uncle and she couldn't have yelled at the uncle for sure.
So , the prosecutor groom would have made a deal with the FL that she would have a contract marriage like thing,and she shouldn't tell anyone about it, especially ML as the prosecutor groom has a knack to irk ML and to degrade ML.The prosecutor groom knew about FL and ML's relationship and wants to devastate ML.As we know the groom is far from innocent.
In return,the prosecutor groom would help FL to solve her Dad's murder case.This may be the 'promise' of the deal and also to keep ML out of this ,not to expose him to danger.
2) The FL has only made a deal.In reality she loves the ML only.As in the bathtub scene ,she was confirming the feelings and future actions of ML.
If she had to betray the ML blindly for the case and deal or money,why would have she asked about what ML would do and gave him a pre -cognition.She would have just straight up betrayed him without confirming his future actions and feelings.
Also from her conversation during bath ,she was crying and telling ML that if it is necessary for him to take revenge like that.From this you can take her attitude in two forms--
(i)She is only taking part in a fake marriage (only on paper and is an act) and she would remain loyal to ML,always his .So she was hinting for ML to wait for her until she clears up the misunderstanding.So that when the deal is completed,she can return to ML ,as it would be illogical and sh*ty of her if she is expecting ML to remain single while she gets married .
Or
(ii)She is a selfish douchebag,who is trying to manipulate ML and just thinks about herself.
Personally,I believe ,option (i) is correct.
3) We cannot see any intimacy between FL and the groom, because there is nothing to start with.They have married only on a 'Deal' and the Groom is not in love with FL.Its only a facade.Also this deal ,benefits Yeom family and step father iL doh.
Also,this makes the Decision of FL to 'not use protection ' during her intercourse with ML ,a possible move by her to cling to ML with his child and she would get a child to show the proof of marriage.
This plot twist makes all the incidents till now plausible.Also there was mention of a dispute between ML and prosecutor groom back in highschool.That may be a trigger.
But, ML is the most pitiful character and from his pov it's still seems a betrayal by lover and everyone is against him.
So, he should go feral and turn the tables so that everyone who let him down would regret and atone.
Still the FL could have given him some hints and made a plan with ML, so that she could have gained a valueable partner in her quest and ML wouldn't have felt devastated and betrayed.