Honestly, I think this is a discussion that will eventually always go nowhere and pretty much always be dictated by personal views unless Capcom says something.
The series was never about romance and romance never played a big part on any game (Steve's teenage crush on the only pretty girl he probably had seen in a long time, probably one sided, doesn't count as romance for me). The series does have a lot of crushes (Carlos on Jill, Ashley on Leon, Steve on Claire, Claire on Neil, Excella on Wesker, Jessica on Chris, Quint on Jessica...this one is a bit creepy though) but most of them seem pretty one sided. Or at least with very little evidence of the crush being two way...maybe with the exception of Jake and Sherry, but then again, Capcom haven't done anything about it, and as far as the information we know, they parted ways at the end of the game.
About Chris. It doesn't matter. RE protagonists aren't normal people. They don't live normal lives. They can be ordered to go anywhere in the world in missions with a high death rate. They work on secret jobs, so they can't share their work life with other people. They know too much, they're scarred and battle weary. I don't think most of them could live a normal life, in universe. So romance isn't in the top of their priorities (Barry is the exception, but he already had a family before the events of the games, and this family is probably the reason he doesn't show up very often. He's a part of the BSAA, but I suppose he doesn't partake in many missions, or probably has some sort of consultant position). Even Chris and Jill. I think they're friends. Best friends. Partners. But I can't see any romantic connection between them except by very sutil hints that might or might not be something. Chris and Piers (to me, they feel like Captain America and Winter Soldier situation. A victim of the ship generation). Same thing. Friendship exists. Not all relationships between two people that care about each other needs to be romantic. But hey, we live in the ship generation so...
As for Leon...ugh. I hope they drop the Ada thing. Again, even in universe, they can't be a couple. Leon's pretty much the same as Chris and Ada is a wanted woman in a lot of places. They can't have a quiet life in a house with a white fence. It doesn't match either character. It's barely romantic at this point. I suppose they have physical attraction on each other, of course, but again, based on what we know, it's not the romantic couple thing. I actually liked the fact that RE6 Leon seemed, at times, a little tired of their cat and mouse bulls***. Yes, he got sad when he thought Ada died, but he refrained of chasing her all the time like he used to do. I actually cracked a smile the first time I've heard him saying to Helena "We stick together" when she urged him to go after Ada once more. Leon is flirty, of course, he strikes me as the 'one night stand' cliché. I do think Leon and Claire could be an interesting couple, because even though she's not in the same line of work as him, she knows a lot too and their connection would be easy. I hope the new Netflix series could show some light on it. Even if to show them as best friends. Leon and Claire were always connected (or the producers wants us to think they are), but their interactions were always so shallow (Degeneration, for example). I liked the flirty tone they got for RE2 Remake, so I hope this stays.
As for their sexuality...I really think it doesn't matter. Unless Capcom says so, it'll be a mystery and one that doesn't bother me.
