Yes it's infidelity
She's a robot, not a person. If you fucked a sex doll it would not be "infidelity" and nor is this. The same exact actions apply to any sex toy. It's ridiculous to imply otherwise. She might feel otherwise about him having sex with anything, especially because of how she reacted to it doing simple things like checking on her daughter, or making food, but that doesn't make it a real person, and it doesn't make it infidelity.
so hardly morally a-okay
You feel the same about vibrators and fleshlights? That's a really odd thing to believe. Do you also think porn is morally wrong? I mean there are actual real people in porn.
In the case of the lad at the party, it's a synth where he wasn't the primary user, grabbed and forced himself on her, and turned her off so she couldn't resist and his actions wouldn't be reported to the primary user.
I'm going to assume based on your tone that you believe she cared at all about this. She was following programming, and didn't give a shit what happened to her. She's a robot, not AI, not a person, nothing. Just programming. It was gross because the boys were being and acting gross. If I took a barbie doll and carved it up and burned it, you would think I was gross and creepy. It's not however wrong to the barbie, and you should feel nothing towards the barbie doll. It's not okay, but the fact that it's a synth isn't the reason at all.
Synth-based sexual consent
Should absolutely and more than likely will come up on the show. They have already explored all kinds of similar things on the show, and the show is literally about AI becoming a reality. Rights, consent, concepts in and around the idea of them doing, feeling, and thinking whatever they want. If the show is any good they will explore every aspect of this, and from what I've seen, it looks like they will.
becomes something not exactly the same as an inanimate
This might be true for how you might feel about a person doing something to it, because it could imply their willingness or feelings towards doing it to a real person, but they are in fact nothing more than robots. Once again, thinking otherwise is ridiculous and actually dangerous for real life. Something looking human does not make it human, not in the slightest. In our lifetimes this will probably come to pass, and it's actually people like you that make me fear what is capable with just something that looks human. The ways other people could use them to trick people, to acquire more from people, and to manipulate people all with a simple robot that just looks like a person.
If the boys at that party display that behaviour towards a synth when drunk, who knows if they'd do that to a real woman?
Yes, this is a proper way of looking at it, but the issue lies with WHY you feel that way. You seem to be looking at the synths like people. The show and movies like this show clearly attempt to make people feel this way, but you really really really shouldn't. It's a robot. Things people might do with a robot, with something not a human, are not always (99.999% of the time) not what they would do with a human being. See things like porn for instance.
If they turned the synth off so it wouldn't know what they were doing, would they spike a girls drink so she passes out
They turned it off because it was going to tell on them. This is not the same as drugging a living human being. Once again, this is why you might look down on someone for their actions, towards anything (like what they might do to a barbie doll) but to act like it's the same is ridiculous. She's not human, you really don't seem to be doing well with that.
Is "it's okay, she's just a synth" a way of justifying actions they know are wrong, like "she was asking for it?" etc.
This one right here is COMPLETELY ridiculous. If I said "It's just a sex doll" when asked why I fuck its ass hole dry when I'm bored, would you assume that this meant I would rape a women? A piece of rubber and a women are not the same thing, not in the slightest. Do you think the same way about porn with regards to this? I think that's pretty odd.
Maybe I am making too much of it
Some of what you're saying is fine, like I said, thinking someone is odd for doing something odd, to ANYTHING, is fine, but the reason you're doing it is because you're giving those robots human characteristics. The show is doing that on purpose, attempting to blur the line, and they use human actors as robots. The TLDR of it is that you're being tricked into thinking the way you are, and it scares me quite a bit. If a TV can do this to you, what if a live robot was in front of you. Would you fight for the rights for robots to not be shut off, just because they are human? What if I put animatronics in my sex doll, and make it say things to me. At what point would you start thinking I was a rapist for fucking a robot that I never had to ask?
far too many grey areas
With the normal synths? No there are not, and that's the problem. With the AI there are, there is a lot to talk about there. We could even talk about, ponder, and even debate if they are more than extremely complicated robots that still do not actually feel anything, but are programmed to react like they do, and then continue to believe and act as though they do. We could even question if that's all we are as people. But to question if doing something to a robot that is nothing more than some lines of code to make it fold laundry, not hurt people, and report crime, is not the same thing ,and you're going so extremely far with it that it's not even funny.
with them being so humanlike and anthropomorphised to make it the same using a vibrator or fleshlight-type toy.
Look up real doll. Do you similarly feel the same way? If I put a voice box in it? If the voice box was connected to sensors along the body with an extremely long list of responses that were catered to every sensor to create a unique experience? Once again, at what level do you start thinking this type of stuff?
Or is it all just because they used human actors and are portraying AI on the show? Are you simply unable to properly process that most of them are just robots that feel and think nothing other than what they are programmed to?