There are several reasons why it made sense to make them a straight couple.
First of all it makes sense from a thematic point of view.They clearly went with an "opposites attract"-theme for the 2,I mean Rakan wears bright colors,is very carefree and happy almost all the time,wheras Xayah wears dark colors,always plans ahead and rather grumpy.The 2 having 2 different genders only highlights this.
Then there is the fact that I'm pretty sure that Riot want Xayah to be the harpy(I know harpies are usually depicted with wings for arms,but they like to mix things up) of this game and harpies are female,they are defined by their gender.And Rakan was made a pheasant,so it made sense for him to be male.
And then there is the fact that ,LoL is a game that is played in a lot of countries,which puts certain restrictions on it.
And there are simply countries who are against homosexual relationships,so if a gay couple would be introduced in the game they'd likely be unable to fully show that in the game.
Just look at the new beauty and the beast movie,that film got a 16+ rating in russia and was outright banned from malaysia because it featured a gay character