Since Rose doesn't answer it, and he kind of twigs on what must follow, he decides to fill in. "He's not gay, Rose."
An engagement certainly explains the ring-- still comes as a huge surprise, though. The Doctor hadn't wanted anything to do with relationships, always made comments about 'domestic', so it's one thing to get the impression they're together... but engaged? "... Wow. Congratulations. Is, uh, that a human side thing kicking in?"
He puts on a faux-innocent face. "Oh, he's tied me down plenty." Before the Doctor can 'OI!' or otherwise comment on that, he quickly continues answering, "And before, actually. It's-- complicated on your end, but we'll explain sometime. And we're married. You should be happy for me! Happy for Ianto, too!"
He gives Ianto another squeeze. "Lead on, cariad. We can sit outside here, enjoy the fresh air."