So the first is Iron Will and Fluttershy enjoying a moment together. Iron tries to impress his wife with his strength when she tries to reach a high shelf. Flutters is amused, not only because he knows she could just fly up there if need be but that he feels like he has to make an excuse to hold her in his arms. She’s fine with being hugged just for the sake of being hugged.
Next up is Sombra and Flutters. Sombra is up late, reading a old tome or something equally ancient and (really) boring to everyone else. Flutters tells him he should get some sleep. As he absently assures her that he will go to bed ‘soon’, Flutters pulls out the big guns.
F: Sombra, are you coming to bed?
S: I shall soon enough. I just have one last chapter to complete and then I shall rest.
F: That’s what you said two hours ago.
S: Yes, but some of the words elude me. Rest, Love and I shall soon join you.
F: … [Draws close though Sombra seems to not notice her approach] Are you sure I can’t… tempt you to come to bed sooner?
S: I really do need to fin- [Turns and stops as he stares at Fluttershy. She looks at him with such… eyes.] Uh…
F: [Innocently] Hm?
S: [Flinches] T-t-tempt me all you wish, Love, but I must complete this chapter. [Turns away, blushing]
F: [Smiles] Alright. [Leans close to whisper in his ear] But don’t stay up too late. I have something I want to… show you later. [walks away, letting her tail brush his flank. He shivers.]
Less than two minutes after this, Sombra rushes off to bed to ‘get a good nights sleep’.
Lastly is Iron cuddling with a Pregnant Sombra, who is both peeved and flustered. It was only after getting to know Iron Will and Fluttershy that he realized how different they showed their love. Fluttershy was subtle, giving him the occasionally nuzzle or kiss when she thought he needed it. Iron, on the other, was very openly affectionate, constantly and randomly embracing Sombra and Fluttershy and showering them with love and kisses. Fluttershy doesn’t mind but Sombra still gets flustered.
I: You okay, darling?
S: I… I am well, husband but I wish to be released.
I: Really? Cause I thought I heard you ask me to hold you.
S: That was hours ago and it was just because of my hormones.
I: Good. How often do I get my grumpy little shadow to jump into my arms?
S: Sir husband!? I assure you, I am not a grumpy little anything! I am the ruler of shadows and darkness! Fears by many and-
I: Lover of cherry flavored jello.
S: It is delicious! Even more so when you add ice cream. And- W-wait, don’t try to distract me! I want you to retract your accusation of this king being a grumpy little anything.
I: Okay. You are aren’t a grumpy little anything. [Smiles] You are my grumpy little husband, beloved by me and our soon to be born child. Better?
S: [Fights the urge to faint from embarrassment. Looks away instead.] M-much better…
Note: Sombra, I feel, is a very traditional guy. He speaks old world, much like Luna though he refers to himself at times as ‘this king’. He also took being pregnant very seriously, choosing to become the house wife rather than risking the life of his child working. While he is assured that working won’t harm the child, he refuses to accept such ‘radical’ ideas and sticks to his ideals. When Fluttershy got preggers, he went full on protective husband, keeping anyone (save her animals cause he trusted them not to harm her) away from her. Had he a horn, it’d have been burning holes through ponies left and right. Instead he just growls and snarls at them.
Unlike her friends, Fluttershy has two husbands. Her first husband is Iron Will, who kept returning to Ponyville to make her feel satisfied. He might not have gotten her to accept his way of being assertive but she was satisfied by his company and companionship. They got married and raised their first child, Ash, whom they had found on their doorstep months before. After that, their first child together was born. Her name was Sea Holly and she was very clingy toward her big brother. Ash never minded, choosing to spoil the filly rotten.
After that, Iron went on a business trip and discovered a nearly dead King Sombra hiding in the frozen north. Not knowing who he was or why his horn was broken, Iron brought him home. Fluttershy was scared at first, remembering how this stallion had tried to destroy them but she also knew he was hurt and needed help. Warily, Fluttershy treated him while Iron acted as her body guard.
TLDR: Sombra is picked up by Iron will, falls for them both and he becomes Fluttershy’s second husband and Iron’s first.