Lies Of Omission
Keiko has only cheated on Miles on two occasions. The first of these barely registers in her litany of things to feel guilty about, but she's never quite got over the fact that she slept with his best friend.
Usually, in the normal run of things, it would have been Keiko who was off the station, Miles who was left with the empty quarters and the lonely bed. But Keiko's come to realise over the years that nothing can be relied upon, and things will always turn out to be more complicated than you expected. Keiko considers herself to be quite the philosopher, really.
Neither instance of adultery has ever been mentioned to Miles. This is despite the fact that she tells everyone how honest their relationship is, that they can tell each other anything. Occasionally, she wonders if she should be worried that Molly didn't seem at all bothered by the fact that Julian had breakfast with them. But Miles is a better person than Keiko, in her mind at least, so she lets the thought stay unconsidered.
Sometimes, when she's feeling old, she remembers putting Molly to bed and sitting down to talk to Julian. And she remembers that, yes, she flirted back and, no, she didn't do anything to put him off when he leaned in and kissed her.
Miles was mentioned only once after that, and she wasn't the one to say his name.
"I don't want Miles to get hurt, ever." Julian looked sincere when he said it, and Keiko remembers thinking that he'd left it a bit late for that, that the time to worry about hurting your best friend was at some point before his wife had her legs round your waist and probably before you'd enquired politely about her contraceptive situation.
If Keiko were the suspicious type, she'd have wondered why Julian was so casual about staying the night, why he didn't seem uncomfortable in a strange bed and why he seemed perfectly content to just drape an arm over her and fall asleep in his best friend's marital bed. Keiko had actually surprised herself by falling asleep as well, despite her expectations of a guilty hour or so feeling guilty and recycling the sheets.
In the morning, she woke up to find that Julian had tidied up the living room and sorted out a surprisingly non-threatening healthy breakfast for Molly. Keiko was surprised by how normal the entire situation felt. He even kissed her goodbye before leaving to heal the sick, and the only thing that threw her was the look of pure, unconcealed envy in his eyes when he looked around the room as he left. "Miles," he said quietly, "Is an incredibly lucky man."
And Keiko smiled, because she knew that it was utterly, unquestionably, true.
She didn't feel guilty until Miles came home. Specifically she didn't feel guilty until he'd settled down next to her on the sofa and asked, inevitably, "So, how was Julian?" She'd paused for a few moments to convince herself that he didn't know, and wasn't in fact casually asking about the sexual abilities of his best friend as experienced by his wife.
"Lonely." Which was true, she assumed. "I think he missed you."
"It's a bit sad, really. Molly said he was round the other day."
"Yes." She'd decided there and then that Miles was never going to find out what she'd done. It wasn't like she was in love with Julian, not like she was going to invite him back for an encore. Miles wouldn't understand, and she couldn't just say "I slept with your best friend, it was great, but I don't love him and I do love you."
The one thing that Keiko holds true above all else is that some things are better left unsaid. She's been lying about the honesty, because it seems like the sort of thing you're supposed to say to people. Sometimes a lie of omission is the best course of action. She assumes the Julian hasn't said anything to Miles, because he's never mentioned it. In her imagination he'd get over it soon enough, sulk for a few days and stop talking to Julian, but he'd never leave her. For all the guilt, she's never felt more secure in her marriage. Miles is an incredibly lucky man - and Keiko is a very lucky woman.