Just a preface: male here, late twenties, heterosexual, married for going on five years to a girl I met in college.
This is going to be a long post, so bear with me...Short answer is: yes they fizzled out after college, but now they are returning.
Growing up as a kid, I always had primarily male friends, and I never had too too many. It was always a pretty tight-knit group. This dynamic somewhat changed during the later years of high school, when my "core group" started befriending more females, and it was a more even distribution (maybe 60/40 male/female split). I remember senior year especially, I would hang out with four people: two males and two females. Granted, I was always closer to my male friends in high school, but I started growing closer to platonic female friends. I was not interested in dating at this point (the two female friends the group gathered was because one of them was an ex with one of my male friends in high school who continued to be friends afterwards). Anyway, I wouldn't say it was a strong female friendship, but I started to realize that platonic female friendships can exist just as easily as male ones.
Fast forward to college. First year, of course, I grow close to a handful of male friends almost immediately. But then about a month or two in I meet my future wife and we start dating and start hanging out with her friends, which merge with my friends. At this point, we have quite a large group of half and half males and females who hang out with each other on a very regular basis. This is the closest I've ever been to that "sitcom" feel you speak of in the OP. We were around each other constantly. This was mostly in freshman and sophomore year.
The later years of college people splintered off quite a bit more. We all still were friends, surely, but smaller groups within the larger group started to form. I still had two or three male friends I'd regularly hang out with, and my girlfriend (now wife) had her three or four female friends she'd hang out with. I was essentially part of both of these groups. There was overlap, as mentioned, we all hung out a lot together in the earlier days, but now as college presses on you get more internalized.
Eventually, the most common group dynamic became myself, my girlfriend, her 3 female friends, and my male best friend (who was extremely close with one of my girlfriend's friends). So a gang where two men were outnumbered by women. We were a pretty tight group, especially in the later years of college. My girlfriend and two of her best friends were roommates from sophomore year to the end of college, and junior year they moved into a house together. A house I ended up essentially living at (I lived on campus, but was at the house all the time). At this point, I effectively became really good friends with all the women even more so than before, so much that I'd call them plus my best male friends my best friends from college. Senior year I bit the bullet and officially moved in with them all, threes company style. One man (me) and three women. Again, these were my best friends in college. My best male friend was over to our house very frequently.
This is the point of my life when I realize I get along with women a lot easier than I get along with men. It's also the point I realize my male best friend in college was gay, for whatever that is worth.
After college, we all moved away and went our separate ways. I married my girlfriend, all our college friends were of course in attendance. We keep in touch, but we don't live in the same city anymore. The friendships never "fizzled out", but they aren't very active since we aren't geographically in the same place.
After we get married, my wife and I move to Austin, TX for her work. At this point, naturally, we start making friends. It becomes clear that the married dynamic for friendship is two-fold: either you are friends with someone of the same-sex or you are friends with a another couple. It becomes increasingly hard for me to become close to male acquaintances, as my bar for friendship is quite high already, and, as I said, it's more difficult for me to get along with men than women, for whatever reason (I could write a whole thread about this). The solid male friendships I do develop form from the mutual couple friends my wife and I share. The dynamic, there, of course, is that that the men are more friendly with each other and the women more friendly with each other, so I never got significantly close to a platonic female friend.
At the beginning of this year, however, I switched jobs. Usually I'm not the greatest at forming workplace friendships, but at this new job I almost immediately click with a female co-worker and we become pretty good friends. A couple months later, another female joins the team and befriends us both fairly quickly. We three start hanging out at work frequently, getting coffee, talking in the break room, etc. At this point at work, I'd consider them both my best friends at the office, and we've begun a friendship outside of work as well. So I am happy to report that female friends are still possible later in life, and I'm glad I've acquired some new ones recently.
However, I will say I can still feel the stigma. Being a married man is also another cog in the gear because people automatically assume the worst. One of my female friends is in a committed relationship with a guy that works at the office (albeit in a different department), so it's less weird, as we'll almost always include him in stuff we do outside of work (which I include my wife in of course). However, the other female friend is single, and fairly young (early-to-mid 20s). And I definitely feel a haze of judgment occasionally when we go out for coffee together or chat together, just the two of us. Which is a shame, because it's the 21st century, and men and woman are allowed to be friends. I fear that these friendships will fizzle out, too, though. Like you said, because of how society views men and women being friends. As people get into committed relationships, room for opposite sex friendships become less and less and the ones that do form are viewed in a skeptical light from the outside world. So it may just be how society works, for now. I do hope one day people can move on from that at some point, but in my experience it still seems to be there.
Anyway, apologizes for the lengthy post. I've had this topic on my mind for a while. It's something I think about since I've realized I'm really no good at converting male acquaintances to friends, they're very closed off all the time and literally never say what they mean. It's so much easier to get to know females, in my experience. And they're way easier to talk to, to boot.