When he walks past me and slaps me on the bottom and says ‘nice arse’ I could honestly smack him in the face
Aaargh! Given how things are between you sexually, that's really tone deaf. If he wanted to turn the relationship more sexual , he would have to be respectful/subtle about it.
This is the impression I got, but feel free to correct me if I'm wrong.
It became a power struggle/he is manipulative about it. You wanted other stuff, but because you didn't 'give' him penetrative sex because you didn't feel able to do it, he refused you what you wanted.
It has become about him controlling you/having his way, and that's why when he does shag you he just sticks his c*ck in you and doesn't consider what might do it for you at all. He resents that you weren't so keen on having sex with him after he gave you an STI while lying and saying he'd been tested.
The 'nice arse' and unwanted slap show that sexually he sees you as an object/chattel rather than a person. He thinks you are his.
I imagine him as having been quite pushy and manipulative, stroppy about it all in the past sometimes.
The sti made me feel dirty. Ridiculous really, they are common enough, but it really really upset me at the time.
You had sex with him based on the belief he had been tested and was clear. Your consent was based on that. He hadn't been tested and wasn't clear. This means that you didn't consent to the sex you'd previously had as your consent was based on a condition which he knowingly didn't adhere to. He effectively raped you, you did not consent to sex with him while infected and he still did it, that's why you feel violated and it's messed you up about sex.