involuntary prudery

More on this so-called volume-four-that-will-never-be-finished-at-this-rate. I decided, mostly just as a writing exercise, that my main characters need a sex scene. They've been living together for, what, three years now? and have an eighteen-month-old daughter, so yeah, they have undoubtedly engaged in this pursuit in the past, but I haven't actually observed them doing it. And I figured it would be good for my knowledge of them to write about it. (Also, having been in escapist mode lately, I've been reading a bunch of fanfiction on the net, and my goodness but people who write fanfic like to write about sex. If you were an alien preparing to visit Earth, you could get all the sex education you needed, and then some, just from reading fanfic. Except you'd end up thinking that about seventy-nine percent of Terran sexual activity was between two men. Which would certainly be interesting, if not strictly accurate. Anyway.) 
The mechanics are not the problem. Sex scene or not, I don't propose to write anything X-rated. (I'm not morally opposed to it in most contexts, it's just not what I like to read, hence not what I like to write.) The thing is, I keep feeling as if I'm invading my characters' privacy. Hey, this thing is private between the two of them, I should get my voyeuristic ass out of there (as it were). Which, since I'm the Author otherwise known as God, is not how I'm supposed to feel. But....he's making love to her, not to me. They don't think anybody is watching them, and it makes me feel guilty that they think they're unobserved and they're not.
How idiotic is that?