As someone who writes a lot of dragon sex scenes: they're hard, even when you know what's going to happen. My advice is, don't talk about penis going into vagina (or whatever is going into wherever) - talk about everything else. How they kiss, hug, lie on the bed together, look at each other. Talk about the emotions, how they touch each other, the way scales feel. The warmth of flesh, the feel of breath against skin. The way their breathing quickens and heartbeat increases. That stuff is hot.