I don't think anyone said anything about having sex everyday is realistic or the key to maintaining romance. I'm going on 10 years and that's definitely not realistic for almost every couple at that stage.
Your note about having a mutually agreeable relationship is very important though.
Expectations should be discussed. I think that's where it can get unhealthy - when everything changes like with kids and one person has different expectations and the communication isn't there for whatever reason. Maybe the wife assumes all is romantic and perfect despite the shit because the man doesn't raise any red flags, they've been together for a long time, and he's a good dad which she finds attractive/sexy. But men tend to not talk about feelings and expectations as much. And for many couples, it harbors resentment and dissatisfaction, often even later (15+ years of marriage) after the kids are gone. It is easy to get consumed by kids; it's an amazing thing.
I'm just saying that I think most men see romance differently from the author and both sides have to put in a conscious effort to maintain the romance, esp. with kids, or at least have a mutual understanding like you mentioned. That and I do think good things happen for everyone (parents & kids) when the husband/wife relationship has more of a focus. Too much helicoptering and slaving going on with parents these days.