Gisele Bündchen's addressing rumors head-on ... claiming she never stepped out on her marriage to Tom Brady -- and, claiming the rumors started 'cause she's a woman.

The model -- who was married to Tom from 2009 to 2022 -- sat down for an interview with The New York Times where she finally addressed claims she cheated on Brady with jiu-jitsu instructor Joaquim Valente.

In the interview, Bündchen straight up calls the claims she cheated lies, and while also saying she's not looking to make her life one big tabloid story.

But, Gisele did offer her theory on why the rumors seem to gather steam ... chalking it up to a sexist attitude toward women in breakups -- claiming ladies always take the blame when they leave unhealthy relationships.

The mother-of-two says the same goes for any woman who breaks up with their man ... it's just amplified with her because of her public platform.

She also confirmed she's dating someone -- though she didn't say Joaquim's name -- adding this relationship is so much different than any of her past couplings ... way more open and honest, according to GB.

We broke the story ... sources close to Tom told us he isn't buying the relationship timeline Gisele's selling -- saying he thinks they started dating way back in 2021.

Meanwhile, Bündchen got really emotional when talking about Tom during an interview with ABC's Robin Roberts earlier this month -- saying she didn't want to divorce Tom, but ultimately needed to make the change.