Right, so this is going to be a headcanon for how the Mane Six go about romantic situations and relationships.
Twilight: Shes a bit oblivious to how relationships and romance works. Shes never had one, and has never pursued one, and wouldn’t know if someone wanted one. Shes read books about them just in case, and if it ever does happen, she’d bring a pocket guide about how to have the perfect first date. We all saw how that went with the sleep over episode.
Applejack: Shes married to her job, to busy for any more than a casual date or two, just for a taste. She doesn’t like it, but anything to keep the farm going on. She hopes that one day, where Apple Bloom is old enough and working full time, that the work spread between all three of them will make it easy enough for all them to find their special somepony.
Rarity: Much like Applejack, married to the job, busy playing mum to Sweetie. However, a lady does have her needs, as you might say. No time for relationships, but shes doesn’t mind having casual sex every now and then to ease some tension, after all, masturbation would be so uncouth.
Pinkie: Friend to all, fillyfriend to no one, yet. I believe demi-sexual is the right word for what Pinkie is, she only forms romantic relationships with people shes well invested in. She doesn’t have her sights or any leads on who that may be, but shes happy just having all the friends she does now.
Fluttershy: She doesn’t really care for it much, one on one dates, she prefers group dates, convince Dashie to come with her on dates, and have her date bring a friend to, that way neither of them were alone.
Rainbow Dash: Her love is racing, sports, et cetra, her partner would need to be someone who loved flying, racing and everything else as much as she does, and be 20% cooler. While she would prefer to have another pegasus, she not apposed to dating an earth pony.