Actually, from what I recall from the episode, Miles wasn't struggling with the idea of sex with Keiko, he was having trouble with the idea of touching her at all. Initially he could not even comfort her the way anyone would comfort a child, which is commonly to embrace them.
Exactly. The dilemma was "Can our relationship survive this?" -- something much more urgent than, "Will I be able to get off tonight?" That was the whole point of -- in the very same scene -- Molly refusing to accept Keiko as her mother. It was about continuing the family, not getting laid.