This might sound strange... writing a romance I started to see romantic potential between my lead and a side character, rather than strictly between the two leads... I'm starting to swerve. How can I suit my writing/mindset to keep the relationship with the side character platonic?
When characters develop minds of their own...
Writing is one of those mediums where people tend to overlook the importance of experimentation. It's seen as a fairly linear process: brainstorm, map the plot, write the draft, edit the draft, publish. If any other type of creation was done this way, most would see the process as incomplete, because experimentation is imperative to creativity. Curiosity is the key to finding satisfaction, and no matter how brilliant you think an idea is at its conception, the best way to do it justice is to question it.
If you find yourself in a situation where your story begins to develop outside your control, don't strangle it back into the shape you imagined for it at first. See where it goes. Let it bleed outside the lines and see what you prefer. You can always return to the original plan. The beauty in fiction is that it has infinite possibility, and if you have the talent to write characters and worlds that determine their own trajectory, enjoy the reward.
In your case, you've designed a romance between two characters but the chemistry of a different pairing has become more compelling, so see what happens. There is a good chance you've simply written a secondary character that serves the story better in a leading role, and there's no harm in experimenting to see if that's correct. If it doesn't work and you're convinced the relationship is more suitable in the platonic category, you will find the reason along the way and that reason will speak for itself as you return to writing the original pairing. If anything, this might become an organic way for you to misdirect the reader in order to make the payoff of your original idea more substantial.
Writing should be an intuitive process. If you're swerving in another direction, satiate your curiosity and then make decisions with all of the information. Just like any other artistic medium, you will only know what's right once you've established what isn't.