I think this is, on the surface, an easy question if we lived in a perfect society. If that were the case, that answer would be; it should not matter who their friends are and I am good with it… If you are in a healthy loving, strong relationship, there is trust, respect and love that provides a protective coating around your relationship.
Now onto the real world. Don’t get me wrong, there are some that have this wonderful bond that is strong and I applaud you and yours for building and maintaining that wonderful relationship. For the rest of us, we have to work on building that foundation and working to get to that level. In the mean time, know who you and your significant other is associating with, both at work and play. There are many less than honorable people who are looking to take what you have; either for themselves or just away from you. Do you trust your significant other, YES, but you should still know and be known to those they associate with on the regular. We all have issues and so do the people we love. If you know that your significant other is a bit self conscious and you are dealing with people that could question their significance, you have to make adjustments to make them feel comfortable until you have that relationship that will stand the tests of time and all that comes with it. This way people know who, what, when and where with regards to you and yours. It is also one more level of accountability by publicly acknowledging your significant other. We all know that today’s society is all about “Me” and “right now” and there are people who would sleep with you, your significant other, take all that they can get and keep right on moving without blinking an eye. So, opposite sex or not, you need to know who your mate is dealing with period and they need to know you.
I believe that at my age (mid-50s), I am not going to find a mate who does not have friends of the opposite sex. Of course, I have friends of the opposite sex as well. The caveat for me is whether or not our “friends” are a secret. If I have a special friend that I talk to or even see from time to time that I don’t want my mate to know about, or vice versa, that’s a problem and that behavior should not exist in what would be a long-term, committed relationship. My requirement (as much as I can require such a thing) is that there are no secrets. If my mate has friends of the opposite sex that are truly “just friends” then those friends should be my friend, too. No secrets.