You could set your story in a historical setting in which the countess and the gardener are truly forbidden from passion, or a fantasy world where the ogopogos and sasquatches are sexy rivals.

The amount of pirates on the covers of romance novels is the direct result of this.

Or just have a lukewarm type of forbidden-ness, like “his family’s greeting-card store is in competition with my family’s greeting-card store” or “we’re coworkers.”

And the hallmark channel answer. I had a coworker who would watch those every single day. I vomit at the visuals (how do they get them so consistent and apparent? You can tell just from the opening shots and title!) of a hallmark to this day.