The author, Diana Gabaldon, says this is the story of a marriage, a deep, close marriage of 50 years or so. Sex, she says, is part of the marriage.
I have read all the books, really loved them, and am enjoying the tv series, which, btw, airs on Starz at 9 pm every Saturday (for the next 6 weeks, There will probably be a break until the next season, already contracted.)
If you can't get Starz, they are available on DVD and Blu-Ray from Amazon and other places.