Is it considered rude to refuse an offer of sexual coupling?

NO. FULL STOP.

The rude thing to do is to assume any offer of sexual coupling will automatically lead to a yes. All sex, if it is truly to be sex, has to be consensual.

Coercion (including cultural coercion—especially the expectation that children and women in particular ought to say yes if an offer is made “politely”) erodes and chips away at real consent. If you can’t say no, or if you’re made to feel uncomfortable or shame for saying no (especially to those with more culturally granted authority), then you can’t really say yes, can you?

Abusers know this and count on it to get away with their abuse. This is typical typecasting fuckery manipulators seem to master effortlessly.

Don’t fall for it. “No” is a complete sentence in every circumstance. You don’t even have to explain yourself.

Let the rape culture proponent implied in your question stew in their own mess.