A "beard" is a partner one has to appear straight to one's peers.

Often done to "keep appearances up" as it were.

Basically in this context they look like a het couple to those looking in but they're both queer and only keeping up appearances for one reason or another.

Yea I think it spawned from Zelda being way taller than Link 💀 and uh Link sure doesn't seem afraid to express a bit of femininity. Furthermore they are just pretty weird and queer overall. Maybe not enough to be trans, but I'll read into it, I'm not shy.