Informed Consent

I can only speak from experience. My (now ex) husband had an affair, I did not know about it, he continued to have sex with me whilst he was having the affair. When I found out about the affair (he left me for the other woman which is when I found out) I felt extremely violated. I certainly would not have consented to having sex with him had I known he was cheating on me, in fact he made it clear that he had ceased feeling any affection for me months before he actually left and when I asked why he continued to have sex with me, his reply was 'because I was horny and you were there'. It made me feel disgusting, truly horrible.

Should it be criminal - I don't know, I imagine it would be hard to prove a case. But I was sexually assaulted in my youth and him having sex with me as if I were an object, made me feel worse than that did, (although obviously the SA was horrible too) damaged me more I would say, and the sexual assault was a criminal offence. But as I say I can only speak for myself and I understand others may view things differently. To me what he did was absolutely non consensual.

I think that you are correct in thinking that in time, laws might change to catch up with certain behaviours that we as a society to not agree with. However, I wouldn't know what changes will be made. Personally I wouldn't class claiming to be single when in reality not, as the same category of not being able to give consent as say someone who had too much alcohol is unable to give consent, however I agree with you that they are both wrong and both victims would feel hurt, if possibly in a different way.
I feel the difficulty lies in how far it can be taken. What about if someone "pretended" to have a different job, for instance, or have different hobbies. Would we then be able to say, I only went to bed with him/ her because I believed they were a dentist who enjoyed classical music? As others have said the cut off has to be somewhere, but where. Cheating can have physical consequences to the other party in the form of being at risk to stds, not being a dentist wouldn't.

For me the question is "is the sex physically damaging or challenging to ones sexual identity"

STDs and aids- physically damaged health.
Using a sex toy without consent is physically damaging.
presenting as a different sex - people have a right to a sexual preference so presenting differently is damaging to ones sense of identity.

Cheating - no, but I think there should be a law around being deceitful when it results in the potential to cause a physical result the person wouldn't have consented to. Like saying you're single, have been checked for STDs or has fertility issues which leads to a decision not using contraception as this could lead to STDs, aids or pregnancy.

I think its whether the lie changes the nature of the sex makes the difference

So lying about contraception use (deliberately, not genuine errors), male vs female, age to a certain extent, known STIs...

Didn't you post almost this exact thing the other day?

There's some case law on it if you want to see what the courts have made of it
en.wikipedia.org/wiki/Rape_by_deception#United_Kingdom

Personally lying about having a girlfriend etc comes under "shitty immoral behaviour" for me, rather than something properly considered a crime.

Seen and been on a few threads about this recently. For me, lying about who you are on a fundamental level is where there is a real problem. So lying about your identity, sexuality.
Also if you are lying about behaviour that puts your sexual partner at risk - so cheating but still having unprotected sex with a partner who believes they are in a monogamous relationship.
These things are serious lies which affect informed consent

Hmmm I think there’s a world of difference between someone lying about whether they are male or female or whether they have a girlfriend or not.

It’s really shitty behaviour either way. But sometimes some shitty behaviour isn’t necessarily criminal I’m afraid. Otherwise where would it stop?