Players can often be toxic for any reason at all. I have about 1600 hours played in competitive. I see toxic behavior in about 25-50% of my games.

I notice that whoever speaks tends to get blamed in some way because usually when people speak up, it's to tell another player to change their hero, or to change their playstyle which is seen as a criticism and instantly causes people to tilt. In all the time I've played, there were only a couple of times when someone was blatantly sexist to a friendly female player just because they made gameplay mistakes.

If you make any comments that could be percieved as criticism, then that person is very likely to retaliate with toxic behavior and if they can tell by your voice that you're female, they might latch onto that as a way to attack you. I suppose that is sexist, but they're using any details they know about you and your playstyle to attack you like they would attack anyone else.

How to deal with it? Be extremely careful what you ask of your team, try not to tell them what they're doing wrong or to change their playstyle. Instead use voice comms strictly for making callouts like announcing cooldowns, announcing enemy positions, and predicting enemy ultimates, and praising the team for any clutch plays. Gamers can be very sensitive and have poor people skills. I wish it wasn't true. Maybe it's an effect of being anonymous. You have to be careful of criticizing them or they will attack you any way they can.