A/N Story heavily inspired by Gokuma's "Staying Behind" series (found at AO3)

Everything wasn't right after Ratchet had returned back among their troops. He did everything to hide the truth, but Optimus, knowing his friend thorough, noticed even the smallest of changes in his demeanor. He had become… reserved, tensed up, and quieter than usual. And more often than once had Optimus observed his doctor staring into the horizon, having not heard a thing that had been said.

The clearest change was in his electromagnetic field, though.

"I'm having a baby." stated the barely audible whisper. The Prime had finally decided to confront his friend alone, and asked about it straightforward. The answer he received thought wasn't at all something he could have been prepared for.
"Ratchet…? What…?" he asked, re-calibrating his audio receptors immediately.

"You heard me!" the doctor snapped, interrupting his leader.

Optimus briefly shook his head, mouth agape, until he finally managed to clear his throat from the sheer astonishment:
"Does this mean…? Is it…?"

Ratchet could pick up on the wishfulness in the Prime's tone. It made giving the answer only that much more painful. He covered his face shaking his head, and left out a heartbreaking whisper:
"…I'm so sorry…"

An ice-cold silence followed, only broken by the almost soundless weeping of Ratchet's, and the quiet humming echoing from either of their frames. Optimus blinked his optics, a palm placed firmly on his forehelm, trying to make something out of the situation. But as he realized his friend's distress, he tried reaching out to him to offer consolation.
"Ratchet…"

But, with zero hesitation, the other bot pushed the intruding limb away.
"No, don't touch me!" he cried out. "I'm filthy! I understand if you never want to see me again!"
The weeping continued, only this time in a higher volume.

Something in the choice of his friend's words got the gears grinding inside the Autobot leader's helm; followed by a horrible premonition suddenly creeping up his spine.

"What… happened?" the Prime asked slowly, his tone dead-serious. He prayed dearly his fear wouldn't turn out to be true. But, it was in vain.

"While… I was prisoner." Ratchet managed to utter from his tears. "M…Megatr…"

The confirmation of his suspicions combined with the mentioning of his nemesis caused the energon boil in Optimus' veins: "MEGATRON… Did this to you?" he raised his voice, his expression sterning out of the agitation and rage. It was a rather rare sight from this particular Prime, who was usually the epitome of calm and stability.

The Autobot medic could only nod miserably.
"I'm so, very sorry…!" was heard in midst of the tears.

Optimus fumed silently for a while, but as it dawned to him how difficult the situation was for his friend, the Prime eventually stopped. He vented air out and back in for a few times before he continued, this time with a steadier voice:

"No… I am not angry at you… Or the child. They hardly chose their parentage. Megatron… He had no right touching you! One day, that I vow, he shall pay for all the pain he has caused!" The Prime pledged, closing his shaking palms into a fist.

Ratchet kept weeping. Optimus stepped closer to the grieving medic and knelt down, enveloping his smaller friend in a tender embrace. After a small break, he continued:
"My spark aches for you. Nobody deserves to experience what you've had to go through."

Keeping his optics locked on Ratchet's face, he reached out a hand to gently, as if asking for permission, rotating his friend's helm back towards his loving gaze.

"But, know this…" he stated, making sure each syllable he pronounced was presented with an equal weight – emphasizing that he meant everything he said from the bottom of his spark, slowly lowering his other palm on Ratchet's abdomen.
"…The sparkling you carry shall equal as my own."

Ratchet suddenly turned his head the rest of the way on his own, his gaze sharpening on the other bot. A completely amazed expression had appeared on his face, as if he wasn't sure if he'd heard everything right.

But Optimus hadn't yet finished. He lifted the palm up from the belly onto the medic's own palm, pressed it tenderly, and continued:

"As… shall you, if you'll only allow it – and this is what I dearly wish – be some day… my beloved sparkmate." he concluded, his gaze – full of love and acceptance – nailed deeply into his astounded friend.

It was then that Ratchet began weeping again, though this time for a different reason.
"Optimus… You don't have to do this." he barely managed to utter, intending to turn his head away again, but this time the Prime didn't allow him do so.

"No." stated the affirming sigh.
"Unfortunately, this is not at all how I would have wished it happening." he confessed.

Optimus closed his palms around his friend's face, keeping his gaze firmly on the other.
"But, I want you to know that my feelings for you are tender, and more profound than all the deepest of the oceanic trenches on this planet. Otherwise, I would not be asking."

He held a small break, lowering his other palm back onto his beloved's abdomen.
"But the fact remains that you and the little one need my aid now, and it is only thus that I rush my intentions. Perhaps one day we shall be able to formalize our bond, and if Primus so wills, to also have young of our own. But this one shall be our first born, and I assure you that these difficult circumstances will not affect my love for you… Or this child. "

It was too beautiful to be true. The medic's optics were leaking lubricant in a constant flow now. He couldn't bear keeping his face turned towards this noblest of all mechs, who now took Ratchet's left hand, entwining his own around it, and continued:
"If you need time, all you need is to ask. And if you refuse, I shall honor your decision. Thus, I question…"

"Oh, Optimus… I don't deserve this!" Ratchet cried out, interrupting his leader's proposal. It didn't shake him one bit, though.

"Thus, I question thee; Ratchet, wilt thou bringeth myself the honor of calling thee my mate?" The Prime concluded, not being able to help but noticing how the face of his beloved was already fighting against a hint of a smile.

Finally, through the uncertain and disbelieving tears, a gentle utterance of laughter was heard:

"…Yes!"