In Frozen, Elsa remained single at the end of the movie, yet she had her sister Anna and was generally happier with her life. Elsa and Anna (who established a relationship with Kristoff, but prioritized her sister's life over her true love's kiss) showed that family is most important and that life is not all about romance. Elsa also showed the world that it is OK to be single and that you do not need romance to be happy; she learned to accept herself for who she is, and romance played no role in that. Giving Elsa a romantic love interest (boyfriend or girlfriend) would ruin these themes, as it would basically say: "forget what the first movie taught, you need romance to be happy." In addition, giving Elsa a romantic love interest (especially a girlfriend) would distract from the main theme of family love, as everyone would be so hyper-focused on the Elsa ship that they will overlook her wonderful relationship with her sister.
If you agree with this and would like to see the aforementioned themes continued in the sequel, then please sign and share my petition to #KeepElsaSingle in Frozen 2: https://www.change.org/p/keep-elsa-strong-and-independent-keepelsasingle.
UPDATE: After much thought, I have decided to disable comments. Normally I would not do this as I despise disabling comments, but because of anxiety I do not want to get into useless debates with close-minded people who refuse to intellectually acknowledge my opinions. Please respect my decision to do this.