Some people see it like that and some seem to identify with men who treat girls like shite and have no regard for their feelings. It's not about Michael not being allowed to like anyone else. It's about the way he's handled it. On their date before Casa Amor he was going on about how great Amber is and how she'll fit right in with his family, they'll love her, etc. Then he jumps ship immediately and instead of manning up and taking full responsibility he turns it around on Amber, invents things and plays the victim card. It's a bit pathetic and weirdly his fanboys lap it all up.
He could have took her aside, told her he'd met someone who he likes and is going to pursue, apologise for hurting her and make it clear things were done and there are no feelings for her in that way anymore. He clearly didn't which is why Joanna had to get the three of them together to try to find out what had actually been said. I agree with you that the girls have been shit-stirring but Michael also has to take responsibility for Joanna having doubts about his intentions.
I'm not buying his excuse that by staying in the villa this is a 'good test' for his relationship with Joanna. If he liked her as much as he claims to he surely would have walked with her but he chose to stay which is another selfish thing because A) you're making Joanna look like a mug sending her home on her own and staying in a villa with your ex and B) it's going to ruin Amber's last two weeks. Even his mates Tommy and Curtis thought he should have left with Joanna yet he's still lingering around being an absolute cnut.
But anyway, I feel like a sad cnut for getting so invested in this show.