Several people already mentioned this since last night, so if you think being the last of her kind as a lesbian is a problem, i have few things to point out :
1- let's not forget we are only assuming this means the end of the line for her race, by looking at this from human's standpoint. however we are looking at a strange race of unknown creatures, we don't know anything about their biology, their reproductive system etc, so who's to say they need a male and female to reproduce? remember Asari from mass effect universe? an all female race that can carry a child mating with another female or male of other races! or Yun from blade and soul universe. how do you even know she needs another person to do it in the first place?
2- If we really want to look deeper at this, being a lesbian isn't an issue here. there has been plenty of lesbians or suspected to be lesbians in history or even now, that carry a child for many reasons. you tell a lesbian you are the last of human kind, you need to reproduce or you are extinct and she says no, that's her choice sure, but she gotta live the rest of her life knowing she was the reason it ended.
3-So you asking why include this in her story? Same reason any champion would flirt with another. being attracted to other people and champions need a reason? problem with this line of thinking is that we put a 'its lesbian thing" filter on it. for all we know she is not compatible with any living race in Runettera now. so she can be curious about men, women, birds, plants, Mordekaiser (if she is into that kind of thing) if she wants but the race is gone!