I think one of the main themes of the RariTwixie love triangle is Rarity finally accepting that she's a lesbian.
I can see there being a joke with Rarity and Twilight having had sex and Rarity coping and saying it doesn't count as being gay, as it was just "experimenting", and Twilight saying it doesn't count as cheating cus Rarity is "straight".
It could also lead to scenarios where both Celestia and Applejack come to terms with their own sexuality as well since Celestia and Applejack both have a link to Rarity and Twilight.
Tamers sets up a lot of plot points beforehand even way early on so I can see him having planned this for a while.
I think the idea of this love triangle as it can really shake up a lot of the stuff in this story as Twilight and Rarity have a link to a lot of relevant ponies involved. It could end up being Tamer's magnum opus when it comes to love triangle dynamics when it comes to both humor, drama, and deeper messages.