In an ideal world things would go as people above have commented, but it's not ideal like that. I knew someone (a cis female) who changed their name while I knew them from their given name to 'Honey' because if you googled their given name it came up with a porn star first. It was a legit reason, but took a long time for people to take her seriously and do it because in their eyes it didn't make sense, just like in her eyes it doesn't make sense that you're changing an already androgynous name into a male name. It makes sense to us because we're used to these things, but she's not. I'm not saying she shouldn't accept you and make the effort to get used to it, but I can sort of see where she's coming from. She could just be reluctant to change because it's a bit awkward, especially if not many other people are using the new name. I felt like that when the girl I knew changed her name, I hate using people's names anyway, and I was never sure which name to call her by because there wasn't a unanimous change over.
I'd just explain that you prefer it, and even if she doesn't accept that straight away she'll soon follow suit when other people are calling you Mac all the time.