There's plenty of things I don't know about that I'm okay with writing. For example, I've never worked at a construction site, but I can make assumptions about how people behave there (some of it taken from TV and movies, which, I'll admit, aren't the best sources of depictions of reality) and I can look up the technical details to make it sound good. It helps that I'm writing for the Doctor Who fandom, because it's very fast and loose with its reality.
However, I've been working on a story in which I want two of the male characters to realize that they're in love, and I have absolutely no idea how to do that. Being a heteroromantic whose gay friends have always been either single or already attached, I don't have any experience here. And it seems so cliche to do that "two people get close, gaze into each other's eyes, and realize there's something deeper there" thing that happens in so many books and movies.
I want the scene to not include sex or physical contact - the scene needs to be the revelation, leaving the reader to imagine that they take things further later. I suppose it would be even worse for me if I did include sex, because I'm absolutely clueless there. I've read some slash erotica, but I've always wondered how much of it is accurate (or possible!) and how much is wishful thinking.
Ah, the difficulties of being a writer!