I wish we could all agree that Ellie kissed another girl out of impulse.
Then we wouldn't have to worry about threads like these.
1. The Last of Us
2. Red Dead Redemption
3. Assassin's Creed II
4. Mass Effect 2
She's not gay, she's just young, alone and looking for someone who will love her. With the threat of the only person she loves about to leave she's unsure how to control her feelings, which is why she says "sorry" after the kiss.
Just young and confused is all she is.