My son is 22 and he is in an established relationship of 2 years and yes he does have female friends from his past, naturally there was mutually supportive behavior during high school years that did not involve dating. This is just the way it is.
I'm seeing someone and I already decided I don't want to have any control over his choices of who he has a relationship with and what it entails. So long as it's not used as a way to hurt me, which some people such as my ex, did use his relationships with OW to hurt, manipulate, deceive me, for purposes of his psychological addiction to doing this (yes, a misogynist.)
If the intention isn't to hurt me, it's unlikely I'm going to be hurt. I have male friends, I go to dinner with them, I dance with them, I discuss things with them. Yes, they would probably like to date me but they understand that it wouldn't work out. I have abnormal needs in terms of being met intellectually, I have tried to partner-down and it didn't work, I tried to partner across into a different field and it didn't work. I tried to partner up when I was younger in terms of age/world experience and that didn't work either (H turned out to be an idiot business wise and alcoholic tendencies as well, whereupon he would tell the same joke to the same people at different dinner parties, that sort of thing...just wrong...and wrong...) Plus he wasn't confident, which made him prey to outside female influences, which in our line of business was a liability. He couldn't tell when he was being played. Thank God he married a Finn who was able to deal with him, handle his drinking, and make sure he paid his child support (she still sends $ every month to our son, who is in college, which isn't necessary, but it sure does help him out...)
Hmmm, maybe I could find myself a middle-aged female Finn and just let myself lose all control of myself...now there's a theory. lol
Honestly, I think people should stay out of other people's relationships. If he is close to you, he will realize you are bothered by what he's doing, and adjust, or attempt to adjust your thinking on the matter, or at least bring his concerns about how what he is doing affects you, to the table, first. If he doesn't do that, then obviously he's not in tune with you and if it's that much of a bother, find someone who will forsake all others in friendships as well as love matches, in favor of you.