One might note that in many countries, cultures, and religions around the World a 20yo man wishing to and actually spending time with women of his own age is considered normal, natural, and a good thing.
One reason this would be considered a positive interaction for two young adults is because this is a very well established path for them to get to know each other and explore the possibly of making a commitment to a life-long loving, kind, and respectful relationship.
Regarding porn, it's porn, and that's not really a parenting issue, that's a society issue, except when it becomes a parenting issue because it's the only way for a young man to express his interest in women.
Rewarding your ability to make your son impotant, I admit I'm not an expert, but what I know about biology and hormones suggests that parenting does not have this effect.
Your son is 20yo. He's your child, but he's no longer a child, which suggests that it's possible that treating him like a child is no longer reasonable parenting.
You can continue to exert control over your 20yo son, and forbid him from spending time with women of his own age. I can't recommend this option, having raised my own son through his early 20's, knowing how important those years were in his life where he was forming the foundation of his future, which included finding the person he married.
I might recommend that you choose to view your son's interest in spending time with women of his own age as a positive step towards him establishing his own family, which, generally speaking, parents tend to view in a very positive light.