As early as ME2 John Shepard asked Mordin to find a cure for Miranda,s sterility. It was finished just before Priority Tuchanka.
After he rescues her and Oriana from Sanctuary, he asks Dr. Michel (Dr. Chakwas was safer with Hackett) to covertly perform the procedure while she's patching Miranda up.
It may be their last chance together so after securing some 20-odd hours of "free time" from Hackett he convinces her to get married in a private, impromptu ceremony aboard the Normandy.
The whole crew attends. Joker officiates. The ring he gives Miranda is slightly hollowed out and contains hair samples from both of them preserved inside.
During the exchange of vows he tells her how much she means to him, in the crapsack world that he has lived in since the Batarians attacked Mindoir her coming became his second lease on life, literally (she resurrected him after all) and figuratively.
He's had friends, and he values his crew like his family, but he never really had anyone to share all of himself with until she came along.
I thought our love would be so beautiful (so beautiful)
Somehow we'd make the whole world bright
I thought our love would be so beautiful
We'd turn the darkness into light
And still my heart is singing, we were right
We were right
And if I never knew you (if I'd never knew you)
I'd have lived my whole life through
Empty as the sky
Never knowing why
Lost forever
If I never knew you, ooh
Yeah (ooh)
Subconsciously Garrus pulls Tali closer.
It was supposed to be just a human ceremony but Garrus and Tali insisted on holding and officiating a Turian and Quarian ceremony. Wrex and Eve are slightly offended and extract a promise from the couple to do them the honor of officiating a krogan one in the future. Not a peep from Liara.
During the reception in the hangar, as a joke Joker arranges for the couple's first dance fully expecting Shep to screw it up but for Miranda to save him.
Turns out Shepard is a terrible dancer when he's alone, but when he's leading and supporting Miranda in a Disney waltz he's friggin amazing and the whole room falls silent as the two dance.
At first Liara can't appreciate the nuances because Asari mindmeld and the physicality is a bit baffling. But when Shep suddenly lifts Miranda by the waist and twirls her around causing both of them to laugh like they're in their own little world, it clicks.
While all are entranced, she grows more melancholic. With every brush and every twirl and every look, tiny waves of jealousy and regret wash over Liara at refusing Shep's offer back in ME2 to join the crew.
No-one in your life is with you constantly,
No-one is completely on your side,
And though I'd move my world to be with him,
Still the gap between us is too wide,
Looking back,
(Looking back)
I could have played it differently,
(I could have played it some other way),
Learned about the man,
Before I fell
(I was just a little careless)
But I was ever so much younger then,
(Maybe, so much younger then)
Now at least I know
I know him well
Wasn't it good?
(Oh so good)
Wasn't he fine?
(Oh so fine)
Isn't it madness,
He won't be mine?
The crew had never seen him so happy. In the few hours that she and Oriana have been there the mood aboard just lifts.
During the brief “honeymoon” in his cabin, Shepard desperately pleads with Miranda to stay safe and under the radar. When she asks why he's suddenly being like this, he answers:
“Because I would let the universe burn for you, Miri.”
Before she leaves she redecorates the Captain's Cabin and leaves a pendant with her holophoto in it. He wears it along with his dog tags.