The show has increasingly evaded the fact that sexual excitement is an inescapable part of Dexter's killing. Despite any lip service otherwise, Dexter's killing is now at worst a supposedly exciting conflict between a nice killer and an icky killer. Or even a supposedly admirable exercise in vigilantism. Even the first season didn't flaunt it in our faces.
Nonetheless, in the first season, Dexter's killing was
his sex life. Rita's sexual withdrawal after life events was in fact part of her appeal for him. When (like the original Dexter character,) she started recovering, she felt confident and sexual enough to give Dexter a blow job. It was a big deal that Dexter could respond appropriately. In later seasons of course, Dexter has cheerleaders thrusting themselves upon him (literally I think in the HS reunion episode, even if ten years later,) and it's no big deal.
So yes, Dexter had sexual feelings for Rita. It was the audience that couldn't cope with marital sexuality I think. But I might be projecting Homeland Season 1, where the hero rejects Morena Baccarin the wife in favor of Claire Danes the lunatic spy. This was so jaw-droppingly wrong
I can only conclude that Hollywood was sure its audience could not possibly regard a wife as sexy, instead of a carrier of tiresome obligations (aka "children.")