My mom met my bestfriend's trans boyfriend...

To add context: I went to the movies with my friend and her boyfriend(who's female to male). And they needed a ride home. So my mom offered them a ride home. After we dropped them off, my mom goes, "So you're friend is dating a trans person." And I replied "Yeah he's trans" and my mom was like "Yeah I know SHE'S trans" and added "I always knew (my bestfriend's name) would come out as gay or lesbian." I ignored this comment, but she decided to go on a rant and made it personal TO ME asking "So do you like girls?" and I replied "No I'm straight, sexual orientation isn't a disease, it doesn't spread." but she replied saying "Well usually people of the same sexual orientation hang out with eachother. I had friends who were lesbian but I didn't REALLY hang out with them much as you do." Basically implying that I'm a lesbian because I have a bestfriend who's dating someone trans. Like I understand she comes from an older generation, but it kind of pissed me off that she was implying that I'm gay because I have a friend who's dating someone from the LGBTQ, what kind of logic is that. She also just said this to scare me away from hanging out with her (because she really never liked my bestfriend). To add on, I don't know why she asked if I liked girls because I'm 18 years old so I'm pretty set in stone about my sexual orientation already.Then when we got home, my mom got out the car and went inside and sat on the couch to watch TV. I got out the car, went to make a sandwich, then walked passed her and she gave me the most DISAPPOINTED face ever. Like she discovered something horrible or that I did something horrible? Like oh wow I went to the movies with a trans person, wow so crazy. I don't do drugs, don't go to parties, and a 4.0 GPA student. Yet this is how I disappoint her, crazy. To make it worse, she invited to drive my brother somewhere, basically meaning she's going to gossip with him about what she discovered about my bestfriend. What should I do or how can I handle this situation?