LOVE is Magic!

Rarity's mind is not involved at all here, which is why she doesn't understand yet what is going on: but both emotions and organs are distinctly playing a part.

The first element is her metaphorical 'heart', which is experiencing the last stages of a sea change in how it views Spike, from poor deluded child who has a crush on her and she has to gently but firmly remind that they are A: not of compatible ages, and B: different species: to a young draconic adult who is well and truly in love with her and would go to the ends of the earth and beyond for her sake: but far more importantly for whom she now feels the same. It will take some days for the full realization of this to percolate up to her conscious mind: but the heart is far less changeable than the head.

The second, catalyzed by the first, is something much more physical, but no less miraculous: it has happened but rarely before, but when a pony and a dragon fall in love, have sexual relations, and copulate at the time when the mare or dragoness is in the fertile stage of her monthly or yearly cycle, it is possible, by the special magical gifts of pony and dragon kind, for them to actually conceive offspring. This is where 'Bat' Pegasai first sprang from, and why, though only the royal sisters of Canterlot remember the reason, Luna favors them as her servants, for, in an age long past, the first progenitors of that breed sprang from her own loins, and a three year lust and love affair with the previous dragon she knew who had been so transformed...

Rarity knows none of this yet, nor does Spike, but they will, in time, and for the first time in her life Rarity is going to get to design a wedding dress of her own!

NOW however, something is about to go drastically WRONG in the brand new couple's love life...