I'm an adult man in his 20s dating a teenage girl
Why are you dating a teenage girl?
Because I think she'll be less likely to cheat on me with another man.
Dating a woman who is significantly younger than you is wrong since there is such a difference in life experience that it makes the relationship very tilted in favor towards the older person. While a small age gap between partners can be okay, a large one is not, especially when the younger person is still a teenager. You should try finding someone to date closer to your own age to avoid having a relationship with a large power imbalance as that is wrong and controlling.
She's very mature for her age, I think it's fine.
Even though she seems mature to you, she's still a teenager and it's inappropriate and even illegal to date someone so young. You should focus on finding a partner closer to your age.