Mate, i know exactly what your going through.
All my friends know that my man is always erect. They even shout out about it that I have a ... when im with a girl etc... I just laugh
My solution..
I'm open about it, if shes kissing you, then she doesn't mind if your getting a boner over her.... I'l kiss her, then look down, or poke it into her as i kiss her, thrust my body into her hard then pull back and say....
"now look what uv done" your a bad influence on me
or something like that... she will laugh.
I always joke with girls about my penis, and girls say that im the horniest guy around as I can keep it up for literally hours. When i kiss a girl il just push it into her, and blame it on her in a joky way. Girls love it. Imagine if the girl got wet as you kissed her, you would love that

just dont let it be a weakness.