Well… first, you don’ t have to have sex in the first chapter, especially if it’s got a strong plot.
And with how I’m doing this rewrite… frequently scenes that were penned under “clean” tend to not get too smutty even when they go to explicit, might have an odd remark to nipples, pubic hairs, todgers, clits, that sort to remind the audience the characters are starkers. Whereas the differences are more pronounced when I write a scene to be explicit, only to realize I kinda need it in the clean too (thus starts a “clean up” where I mark off the explicit bits).
When I view my own stories, the first chapters very rarely have any real sex scenes...nudity or skimpy wear being more common. Actual sex usually comes in later chapters.