I'm a bisexual woman with an ftm partner. I hope this is useful.

Just ask your partner. They may not be sure tho. In general, I tend not to start/participate in arguments with people about stuff like this, especially in person. And I've generally been cultivating some inner strength for myself so I'll be able to show up for my partner when things like this happen.

It hit me recently how vicious transphobia can be. It's subtle, like I'm sure people asking you if you're not just a straight couple don't realise that they're being transphobic. But over time, it can have a bad impact on your well being (you and the partner). So best thing for me has been to find resources that help me show up for my partner and for myself. It can be things like having an understanding friend who gets both of you. Just educating yourself, Finding community whether irl or online, etc.