It feels amazing and exciting and sexy and terrifying all at once and all that is overriding any scrap of rationality I've got left
It's adrenaline. Like when you abseil or jump out of planes or anything else that RATIONALLY is a stupid decision. This is, too.
It's a secret, and it's passionate, and it's given you something to obsess over. If you actually met him, it would very likely be a complete come-down, because at the moment you can both role-play - he can be romantic and interesting and woo you, without any regard for who he actually is or what he is actually doing. He can be texting you on the toilet wearing three-day-old socks and telling you he's mad with lust. You can do the same.
It ends, though. Whatever happens, the body doesn't respond like this forever. It can be exciting and arenaline filled now, but it will start to lapse, and you'll do anything possible to get that thrill back. Maybe behaving a bit more risque, maybe meeting him, maybe a full-blown affair.
You are likely to hurt yourselves, each other and your partners. Everyone always thinks they will avoid the devastation, very rarely does anyone actually do so. You would be wise to accept this and decide if the current adrenaline levels - knowing that's what it is, hormones, rather than him - are worth that destruction to you.