I’m so WEAK for these two aaa, my guilty pleasure ship qvq, their story contains some more mature themes, so this works as a disclaimer!
 
Women and men, we are the same  
But love will always be a game  
A human vulnerability  
Doesn’t mean that I am weakBlack Sparkle 1
 
Rarity was the seamstress of the court, and Tirek the co-ruler of Equestria (basically what Luna did)
 
Tirek and Rarity’s relationship started as a playful thing, Tirek flirted with her and Rarity(she thought she could get a little bit of information from him to help Twi)followed his game.It was clear that Tirek was only intenterest in her good looks, but Rarity wasn’t just a pretty face. No ties and no pressure into the relationship, they began to slept with each other, Tirek had other mares to toy with (Rarity was also guilty) but he respected Rarity, he liked her charisma.
 
Eventually they growed closer and closer, Tirek realizing how strong the mare actually was, he had him wraped around his hoof.One night when they were tangled in his bed he proposed to her. Rarity walked down the aisle feeling a little bit guilty, this was one of the reasons she and her friends had lost everything…but they loved each other.