Hey, I’m a bit late here so my apologies but here’s my view.

Before I begin I will mention that neither Sex/Nudity nor “killing people” are particularly important parts of the show. The important parts are the characters and the conspiracy they have been dragged into, everything else is kinda just an aspect of those two parts.

I personally think Sex & Nudity should ONLY be in a Film or TV Show if it adds a new layer to it or expands upon an already existing one, such as the plot, character relationships, etc.

I think with Ian and Becky, it’s really just to advance their relationship more, they're not in the “talking phase” or anything like that. They hit it off with each other when they first meet and are both deeply interested in the other. They don’t need to have have sex (and in fact they don’t, they give up because they’re so drunk) but they manage to laugh about it which further strengthens their relationship, the scene also just works as cringe humour and then it’s kinda wholesome when they just laugh it off. But the point is that you need to know that they have just advanced their relationship about 10x faster than most people, but you need them to reach that stage to better understand Becky’s jealousy when Jessica appears.

As for Ian and Jessica, we already know that Jessica is pretty infatuated with Ian but she has been sexually repressed her whole life due to living on the run. When she first meets Ian she tries to show this but she doesn’t know how, and is scared too “make a move” so she distances herself from the group. In season 2, she had just been through an even worse hell than the one she was living prior to season 1. She was probably surprised she was even alive and after a near death experience many people re-evaluate what is important to them and I think because she knew that they now had Janus she decided the next time she saw him would be now or never. She also watched a couple fully making out after she escaped which might have been her sexual awakening, probably 20 years after most people.

These scenes may also not be necessary but at least there is some sort of context and meaning to them, think about the show, the plot, the world itself… then ask yourself when, where and why would they need a sex scene? Anyone with a brain should be able to come to a quick answer to that question.

Last thing, Why is this something that has (I’m assuming) particularly bothered you? Just because overpopulation and infertility are an important part of the story does not mean that just because you can connect them to Sex, there should be more sex scenes, like what!?! You could say that about anything, for example, “It’s weird that this film doesn’t have sex in it seeing as they live on earth, and we know that people on earth have sex” or “It’s weird that this film has no one naked in it when we know they would’ve had to get a bath/shower at some point”. As you said yourself, it’s not important.