Consider yourself one lucky guy, not all women love getting their assholes tongue fucked! I love it personally, and nothing hotter than a hot, shared wife who will bend over, spred her ass wide open and invite me to feast on that tasty rosebud!!!
Sounds like sharing is something he really enjoys, I can totally understand how really getting turned on by sharing would be the key. Prehaps he is a cuck and enjoys that aspect of it, but it sounds like he really enjoys sharing you...