Do actors that take part in nudity/sex scenes get paid more for those scenes? I would think for an ordinary person such scenes might impose a different level of discomfort and stress, so are those rewarded specially?
Generally this varies by actor or actress. Sex scenes get a very high level of negotiation, specifically detailing how much skin is shown, how long, what, when, etc. If they have a scene it's part of their negotiation.
Background actors (aka extras) tend not to be paid more. SAG rules and industry standards do not pay extra for nude extras. But one recent example of them being paid more was Westworld, which had an abnormally explicit basically X-Rated scene in October:
But extras on “Westworld” — as many as 50 of them — were said to have pocketed up to $600 for the steamy session because of the unprecedented amount of simulated sex the extras were being asked to perform, sources said.
Still, if any of those "assorted acts" verge on stunt work — as seems within the realm of possibility — the union contract may entitle the actor to be upgraded to principal performer, which brings with it residuals and a boost in pay.
The union agreement also requires additional payment when a background performer is asked to bring specified wardrobe. However, no additional payment is required when the performer is told to bring no clothing whatsoever.
An SAG background actor doing nudity wouldn't be paid more than normal. A nude body double though is supposed to get at least the principal performer day rate (funny enough, same stipulation as any extra that has a squib, an exploding blood back.)