rainbow dash has always prided herself in being epic, cool, unphased, stoic, and that often reflects in her relationships - and that includes twilight.  
when they first start as a couple, rainbow is the more dominant one and takes the initiative. twilight isnt all that experienced, and usually needed guidance and someone to make the first step - but then one day, twilight gets on top, and starts taking control, and - oh god no, why does she like it so much
 
follow up later that day. how could an egghead fluster her so much. what is this sorcery