It look normal to me, if your sti test come out clean then everything is fine. Does it itch? If it’s itchy then it’s a yeast infection.
Your penile skin should be able to deal with the rigours of sexual activity.
What do you apply to your penis for sex - lubes of any kind, other stuff?
Does your GF use a lube?
Do you have a clean up routine following sex where yo shower and use lots of shower gel on your penis, anything like that?