Why do people think that The Mona Lisa was a mistress of Leonardo da Vinci?

Maybe because he spent so much time with her. I believe it took him around 4 years to complete the painting of her and during that time, she had to sit for several hours as his model each day. However, from most of the information I collected about Leonardo, he was gay. So was Michelangelo and most artists during that time in history, around the 15th-16th centuries or the Renaissance. I think mainly because most famous artists during that time were commissioned by Popes in large cathedrals to do their artworks. There were no women in these churches. Only men. and their works sometimes took them years to complete.That's no excuse, I know, but from what I gathered, this was one explanation for their behavior.