I imagine that pregnancy and organic-style reproduction was a thing among Transformers millions upon millions of years ago, but once they got the Allspark up and running they kind of abandoned it. In fact, most of the documents that pertained to this type of reproduction were destroyed in the Great Purge, and only the oldest of the oldest bots, like Alpha Trion, have ever even heard of it.
They still have "sticky" sexual arrays, of course, but their only use is for recreation.
Then the war happened, the Allspark was lost, and everything went to scrap.
Once the war was over, the missing Allspark became a really, really big problem, because protoforms are useless without the Allspark to give life to them.
That's when the millions of years old pregnancy programming kicks in, and bots everywhere are randomly seized with a desperate, intense urge to frag or get fragged immediately. Their internal temperature rises to dangerous levels, and the only thing that relieves this mysterious new ailment is to have a certain amount of transfluid in the valve.
After a while, medics are surprised to discover that a tiny protoform is growing inside some bots… a protoform with a spark, no less. This is huge news. Now, Cybertron can be repopulated with pre-sparked protoforms, and the Allspark is no longer needed.
Eventually, heat suppressants and a method of birth control are created and generally become available to most Autobots. The heat coding can't be turned off without practically lobotomizing the bot, so it's kinda something they have to live with.
HOWEVER, it's important to note that Decepticons and warframes, while they do have arrays, don't have forges (wombs). They also don't have heats, and Shockwave, while going undercover at Autobot Boot Camp, realizes this far too late.
But he can't really code himself into being able to get heats, and he is a good actor but not "faking a heat" good. So when Bumblebee asks why Longarm hasnt been in heat yet, Longarm replies its because his cycle just finished before he came to Boot Camp.
"Oh," so Bumblebee says, "you'll be due in about… 40 stellar cycles, then, huh? That overlaps with my heat. We should partner up." (Bumblebee's already thinking about if Longarm's dick can stretch like his limbs can).
"That sounds delightful," Longarm lies through his teeth. (Internally, he's sweating bullets and trying to figure out what to do).
A few stellar cycles later, Ironhide goes into heat and gets clanged against the wall by Wasp. Longarm's never actually seen an Autobot in heat before, and it's… something.
Bots are very loud, very desperate, and very unconcerned about who sees them interfacing. And they go all day, and most of the night. Longarm has to recharge to the sound of metal on metal and formulate how he acts when his heat will supposedly hit.
Thankfully, Bumblebee goes into heat before Longarm is due, and he helps Bee with it. He's not prepared for how much stamina Bee has, nor how many rounds he's willing to go for (which is a lot). Longarm has to beg Ironhide to take over for him, lest he be completely drained of transfluid.
When it comes to his own (fake) heat, Longarm tries his best to emulate Bumblebee. The size difference means Bee has to go for quite a few rounds before Longarm's valve is full, and Longarm feels like a shriveled-up husk by the time it's finally over.
