On top of what's always been said, I have some thoughts. You might be offended by them, but that's not my intention. What I'm about to say is based on experience.

To keep things simple, I'm going to use the term "woman" instead of saying "girlfriend or wife" and "man" or "guy" in place of "boyfriend or husband." I don't like using "partner" as a substitute.

I've seen many people in relationships and marriages either monopolize their partner or allow themselves to be monopolized. This is a psychology that doesn't come naturally to me since I'm particularly stubborn and individual-minded. Most of my male friends, once they are in a relationship and especially if they are married, allow their relationship to take over every aspect of their social lives. It's one thing for a man to be spending a lot of time with their woman and going on many social outings together, but the healthy boundary (IMO) ends where both parties must be involved with everything.

For example, I've known many guys who believe that their woman should become a de facto member of his male friends circle; while I have known women who can absolutely be "one of the boys," that is more often not the case; by doing so, the man may be uninviting himself to future outings by his male friend group without even realizing it. I love women, but there are some topics and issues that men need to be free to hash out man-to-man without being subtly chastised by a woman who likely doesn't understand things about the male perspective. The more subtle effect is that the man has less quality time to allow him to feel close to other men, and he may not even realize it as it's happening. The same is of course true with the sexes swapped. In any case, the hypothetical man in this scenario may, at 30 years old, find himself wondering why he has no regular friends anymore; chances are some of his friends found other new friends, and others of his friends became monopolized.

In a similar fashion, something I've seen women do is have their man in tow for things that obviously an average straight male would have no interest in. An example is how it's become popular for pregnant women to have multiple baby showers: one that's girls-only, and another for both girls and guys. I'm not saying that some men wouldn't want to participate in a baby shower, but let's be honest, most men don't identify with baby showers. In my limited experience, these celebrations end up as the men in one room and the women in the other, and virtually none of the men actually want to be there. My point about this is that the way I see women monopolizing their men often involves bringing them into feminine spaces they probably won't feel comfortable with, whereas usually men are either more accommodating if a woman is in the group or simply not invite women to things they know they will not be interested in.

To summarize this point, I see many men and women as not respecting a healthy level of autonomy in the other person, and that women respect the time of their men even less than the men do of their women.

I don't believe that making friends in adulthood is as difficult as people make it out to be. Yes, it's more difficult, but I'm not a fan of the excuses people make in this regard. When I was a kid, I was emotionally stunted, had few friends, spent some years with no friends at all, and ended up "walking the desert" for some time after college. Yet somehow I still ended up with friends at 34. I'm not going to ramble about how I approached adult friendship, but I can summarize it by saying that, if you want friends, practice being friendly, keep promises, refrain from being judgmental, think about what the other person wants, and just get out there.

What keeps many people back, I think, is this idea that being by yourself while in a relationship is awkward or wrong. I think this affects men a lot more than women. If they can be spending time with a woman, the pseudological part of the male brain informs them that they should be spending time with their woman. You absolutely should be spending time with your man or woman, but not at the expense with having bonds with others. I would never expect my girlfriend to not spend time with her friends, and it seems to be more acceptable today not only for women to spend a lot of time with their friends, but also male friends. In such a case, I think women should be conscientious about whether their man is getting enough friend-time; if you know he has friends but he's not nursing those friendships, tell him to go out and play billiards with the boys. Go with him if you like, but also allow him times to go by himself. If you don't trust him to be out having fun by himself, then you have a relationship problem you need to discuss with him.