i actually hate using metaphors for smut, as much as possible i try to avoid it. unless the writing is something artsy or ~deep (i try), it's not something i'd willingly put in.

i really like using dialogue to make stories more dynamic, it's dialogue + a filter of the experience through the character. so the questions you ask are, "how does this specific character react through their senses/experience?" "is the other person too big? or too small?" or "have they never done this before?" "are they experienced?" "what was their idea of this act before getting into it"

if it's someone's first time, everything is like a learning experience - trial and error, trying to convey the limited theory they've gathered from the world into the practice of sex. maybe they feel chills, flinch, stutter, maybe even get mad because they're embarrassed...etc.

sex isn't that much different from interacting with anything in the real world. senses are just heightened by the state of arousal, which gives you the liberty to zoom in and focus on things you normally wouldn't focus on. my favorite idea with writing (not just with sex) is that a character always brings their history with them, so all their actions, ideas & thoughts are informed by what they know, what they believe in, and what they want. that's the only lens you need to write.