Spoiler
Chase is gay even when Sam's influence was its weakest and there's no indication he wasn't. Leo isn't in love with Chase or the thing possessing him. He's in love with the Chase in his memories which Echo made an image of to manipulate Leo.
Oh? Where is that implied?
spoiler
I admit I could have been misunderstanding things in Echo, but I thought it was implied that when Chase was freed from possession he was a totally different person that Leo wasn't interested in anymore. Again I admit I might have misunderstood stuff, but this ruins my fanfiction as the difference between possessed Chase and himself Chase was an important part!