The connotation of boi implies that the one saying bilf is not a boi. Or in other words is not straight.
So basically, when a "straight" guy refers to a gay guy as someone the former would like to have sex with.
This is a definition that breaks gender norms and has been used among straight buddies, and has been accepted as a norm, regardless of its context, sarcastic or serious.
Steve: So he stole all your shit?
Steve: What a bilf
You bilf nugget
you bilfing moron.
I'm gonna fill you with so much bilf you're going to get swimmers ear.