Do you feel jealous if your partner hang out with a friend of the opposite sex?

I wouldn't date someone long enough to consider them my "partner" if they had close friends of the opposite sex.

Sociologically speaking, it's possible for men and women to be "just friends."
There are social institutions and norms and boundaries in place where groups enforce rules of individual behavior and set expectations.

Psychologically and biologically speaking men and women can't really be friends.
For heterosexuals cross sex relationships develop naturally using specific chemicals that are different than those for same sex relationships.

There's a difference between sociologically determined relationships.
e.g. you're in school and your teacher says "you and you, you're lab partners for the next assignment," or you assigned seat sit next to each other in class all semester, or you're both the new hire and your cubicles are next to each other. Then you're civil/friendly towards each other, and maybe you don't have any other friends in school/work, so you start hanging out together, and you "bond" as a reaction for support/protection against the larger group you find yourselves having to participate in.
It's a forced relationship that turns into mutual social benefit, boundaries, expectations, rules, set by the larger group that you adhere to in order to get along and survive.

If you're finding your own "friends," like you dated but it just wasn't right, or there were not enough "tingles" to perpetuate it so you stop dating and remain "just friends." Even if it was for "just friends" from the start, or at the end of dating. It's doesn't really matter.
You approached/accepted an approach based on physiological factors.

You read the recent articles regarding judges handing out different sentences based on levels of attractiveness?
Or seen the studies regarding hiring practices for people that are more attractive than others?
People do it without realizing it.
People have implicit bias based on attraction. Thank puberty for completely changing your perceptions of people. All relationships pursued are based on an attraction bias.
Attraction engages different natural biological reactions that affect behavior and bonding.

"Love" is a biological process.
If the other person has already partially bonded with someone else, and they are orbiting as a "just friend," you are never really in a one on one relationship with them.
Your relationship is partially based on their relationship with their other partner. What affects their relationship, will ultimately affect yours.
You will not completely pair bond with them, as their partial pair bonding attempts with the other person will interfere.

You have become the second husband or the second wife. Or they have, if they make the "just friends!" after you started dating them.

It's kinda like asking "does your boss get jealous if you find a second full time job with their competitor?"
Sure, it's "possible" for someone to be completely ethical and not use tools from one for another, and they may favor one over the other, keep them completely separate.

But for the vast majority of people, they can't.
At best they delude themselves and rationalize it mostly because they enjoy the income and benefits.
And you won't really be able to find this out until you've been with them for a long time, and only if you really know how to pay attention.
And that's a big problem with attraction and part of the process. It gives you a warped sense of reality. You overlook things you shouldn't, and focus on things that validate your bias.

I would prefer to avoid the rationalizing and delusion and moronic pitfalls that almost always arise.
So, if I see/find out they're "just friends" with someone of the opposite sex?
I move on to someone else.

Not at all. I have male friends. Besides, it's not even about the gender; it's about trust and how secure I feel in the relationship. I will never tell anyone with whom they can be friends with. That's silly and immature. Besides, this friend was in your partner's life BEFORE you. If you have a problem with this, don't get involved. Nothing good ever comes out of jealousy and friends will NOT go away, nor should they.

Well said feely, I couldn't agree more!

I would not give a flying duck

I have always had more male friends than female.. And yes just friends..

It may be an issue if they hang out with them more then they do with the one they are dating.. But other than that, it should not be a issue..

Jealousy will kill any Relationship~~~

Sorry if anyone I was to start dating told me I was not allowed to be around some of my friends I would tell them there is the door don't let it hit ya in the azz on the way out..~~~

It depends on her relationship with the person, i'm cool if he's a just friend

I’ve said before on here, my best friend is male and my ex partners have never had a problem with us hanging out together.

I would not like it, no. It could and likely would be a deal-breaker.
I'm quite allergic to men with female friends. I haven't had good experiences with that and some things you do not get over, and shouldn't get over. Instead you set a new boundary of what's acceptable for you.
This is one such thing for me.