I honestly don't see what's wrong with fade-to-black, if that's in keeping with the tone of the story. In my next book (the one in my present avatar, see left!), set in the late 1930's, the couple don't do anything more that kiss, even though they are commited to each other and in love for the first half of the book, when she's 16 - 19 and he's a gentleman.
Then "things happen" and they are separated for many years. They meet again when she is in her early 30's, now a widow. They seize the day. Then enter the bedroom, fade to black. No details. I don't see the need. It's about their feelings, not about what they actually DO. It's about the consummating a love which has been brewing for many years; and as the bedroom thing is a one-off (they won't ever meet again) emotions are running high. A lot can be done with this, through dialogie, little gestures, words. I know your book might be very different, but I think it all depends. Describing "the act" in my book's case would seem almost a violation of their privacy - especially since it's told in the first person, her voice. I can't really see her exposing exactly what they did to anyone. She'd be subtle, sensitive, a bit prudish, and that's fine for the era.
ETA: I should mention that I'm one of those squeamish readres who skip blow-by-blow sex scenes when I meet them in a book that otherwise focused on love and character development. If I've come to care about the woman, it's as if she's a close friend, and "watching" her having sex would be like coming across one of my real-life friends in a private moment. I'd turn away in real life, and I turn away in fiction!