The husband's only "sin" was marrying a Lesbian. It's not really his fault, that he didn't notice as heteronormative life can blind people to the plainly obvious.
Perhaps, he believes that his wife doesn't enjoy sex because of his own failings (which he would never admit to).
There is also the fact that Yukako didn't peg to the fact that SHE is a Lesbian. She kind of dreads her husband's touch. She's trained herself to react to him, to please him, but the charade is starting to break down.
I think Yukako needs a reckoning. Her cheating on her husband isn't fair to him. Her dragging Chinami into it isn't fair to her. Yukako was the one who broke their relationship off, so she could be "normal." Now, she regrets it.
There needs to be a lot more truth-telling and a lot less sneaking around. I also get the feeling that Chinami is not at all happy about being "The Other Woman."