Show is cannon and they make it pretty clear she is straight. Basing her orientation off of beauty alone is vapid to say the least and though rarity is fashionable, she is however not vain to the point of conceit. She was after her "prince" at the galla (blue blood), then on to trenderhoof. She even swooned a little during SSCS6000 towards flim and flam...though the later may have been swooning to the idea of cider as I mentioned earlier.
Rarity is classy, beautiful, not self absorbed, want the world to have beauty, is generous about it, and is clearly straight.
Sorry to burst the head cannon!
Edited for further musings:
Also consider this. Every mane six fandom has a portion that ships their character with another of hte main six and many insinuate similiar concepts of not excluding either gender. My scientist mode is now going to show. Based off of statistics and the head cannon of many fans, not all of the mane six could be bi/lesbian as LGBT still represents a statistical minority as continued research shows that such orientations are not boiling down to choice but are connected to possible estrogen and progesterone imbalances possibly linked to feminine alleles passed down maternally from mother showing increased excreation of the aforementioned hormones and thus leading to a slight increase in the possibility of a same-sex orientation.
Thus logically it follows that based off of statistical average and minority occurance of bi/lesbian traits, the possible allele connection and Rarities parents showing neither hyper-feminine traits (males can show this trait too), and the recorded behavior of Rarity in past events it concludes that Rarity highly probable of being straight.
Okay, science Striker is done.