To all writers of Transformers romance, particularly ones that have cross faction pairings...
... Do any of you people realize how complicated something like this would be? Starscream or Megatron can't just shack up with an Autobot and have there be no sorts of consequences. Let's look at the one cross faction pairing that's canon. Silverbolt and Blackarachnia. That was a big messy business, and even after she changed sides, it wasn't perfect. There was a lot of difficulty over the strains put on their relationship both as a couple, and in regards to their team-mates.
Secondly, people who put leaders like Galvatron, Megatron, Optimus, Rodimus, or any other prominent faction commander or member into one of these things... lord in heaven, take what I said above and make it a gazillion times worse. Megatron and Optimus, together?! Do you realize what that would do? To BOTH factions? I'm not talking a soap opera gasp moment here people. This would NOT be pretty. It'd shake both factions to their CORE if something like that happened. It'd potentially tear loyalties apart, cause schisms, so on and so forth. This is not a simple romance here.
And speaking of Optimus/Megatron... if you write it now, write it in the present. They were never ex-lovers, especially in G1 continuity.
Because, ya know, the first time Optimus and Megatron met, Optimus was named Orion Pax. And Megatron shot him.