This is an issue that comes up for me frequently on flickr, but is applicable to many other contexts besides. As a sort of introduction, let me say this: there is tons of porn on the internet. I don't have a problem with that fact, obviously - I think porn, in general, is great. (And anyway, great or not, sex is a huge and important part of most people's lives, so trying to get rid of it is futile).
The way porn is mostly dealt with on the internet, however, is that it is (usually) placed behind a sort of filter. Search engines often have "safety" or "content" filters of this sort that mostly prevent porn from being displayed (much more than anything else - such as hate speech, violence etc. - which is also reflected in the law via the concept of obscenity; the idea that there is speech that is just so offensive it doesn't deserve to be protected by the first amendment, and that for some reason this speech is necessarily of the "prurient" (that is to say, appealing to one's sexual desires) variety).
Porn sites, on the other hand - dedicated as they are to sexual media - instead of filtering it out, generally provide a sort of warning before you get into the meat of the material. This, incidentally, also provides a sort of legal protection, as minors (persons under the age of majority, frequently and in the case of the United States designated as 18) are technically not allowed to view porn, even where no law exists barring them from actually bumping uglies themselves, as opposed to watching someone else do it (which is a whole different discussion).
Meanwhile, people on message boards, blogs, discussion sites and things of that sort often (as rules of social etiquette deem it polite) provide NSFW ("not safe for work") warnings when providing links to material of a sexual nature (and frequently also material of a nonsexual nature that nevertheless contains nudity), seeing as browsing material of that sort can get a person in hot water while at work, or in certain company (e.g., when kids or girlfriends or conservative friends or whatever are in the room).
All of this contributes to the compartmentalization of sex in media and as a discussion topic, much as the actual act of sex is confined to private quarters and generally not put on display in public or in front of non-participants as a social rule. And despite what you may be expecting me to say by this point, I actually don't have a huge problem with any of that. I harbor fantasies of an alternate reality where perhaps sex is as casual an activity as frisbee, as common a topic of conversation as the weather, or as popular a subject of art as flowers, but realistically, I know that most people aren't that comfortable with sex (though, in a vicious cycle, their discomfort makes me more uncomfortable than I'd like to be), and the last thing I want to do is get in somebody's face and expose them to something that is extremely uncomfortable for them (even if that is sometimes necessary, under the right conditions, for social change).
But, here's what actually does bother me. It's the expectation that, despite the fact that we are all sexual organisms, and most of us have sexual desires, we almost have to pretend like we're not, and make sure that that fact is hidden and ignored as much as possible. Sure, we joke about sex among friends, but generally in the company of family, or among professionals, the very insinuation that we have a sex life is not only undesired, but can have disastrous consequences, in the right (wrong?) situation.
As pertains to my usage of flickr, I'm constantly aware of the dichotomy between the "porn" I post, and everything else (like, you know, pictures of flowers and things). The porn is hidden behind a safety filter, which is great, because it means people who don't want to see it can still look at my flowers and whatnot. Theoretically, I don't need to maintain alternate accounts - one for porn, and the other for everything else - and I'd really prefer not to have to juggle multiple accounts (I have enough accounts to keep track of on the internet between websites not to have to worry about multiple accounts per website!).
But in reality, I frequently fear the potential overlap that may occur by not keeping - in certain words - the sex in the bedroom. Other, more conservative flickr members have noted that once you open your house to perverts, they'll take it over. In a sense this is true, as though perverts usually aren't interested in pictures of flowers, you will occasionally hear the raunchy dialogue drifting out of the bedroom and into the living room (if I may stretch the metaphor further). This is especially true (and especially problematic) when you have pictures (like of family, friends, or models) that might appeal to the perverts, but for reasons they may or may not be privy to, a sexual comment on which might be extremely uncomfortable.
As it stands (and as much as I hate to admit it, as it gives a bad reputation to perverts - although 95% of anything is crap, and sex is as popular among morons and ingrates as it is among the polite and well-educated; and what's more, the former is more likely to be open about that fact, given the reputation it brings - which is something I'd dearly like to see change), but many perverts lack tact, and have a poor sense of social context. The widely anonymous, context-fluid internet atmosphere makes this even worse. It takes a bit of subtlety for a person to recognize that just because a person posts sexy pictures of himself and appreciates raunchy comments on those pictures, he may have a different opinion about comments on pictures he posts of his sister-in-law (who herself may have a very different opinion on getting "sexual compliments" - from creepy strangers on the internet, no less - as he does).
Personally, I think this is all pretty basic level human interaction stuff, but you'd be surprised (or maybe not) how many people don't pick up on that. Hence the "need" to further compartmentalize sex, with a "better safe than sorry" mentality, to account for the morons (as well as the well-meaning snafus, which do occur). Of course, this irritates me, because it feels like I'm being forced to go out of my way to cater to the "differently advantaged", instead of expecting better of them, and forcing them to go out of their way to cater to a more civilized mode of human interaction.
But there's another element to all of this, and it's this assumption that people who get squicky about sex (mention of sex, acknowledgement of sex) are entitled to their being offended, and us better adapted to the truth of life and its sexual connotations (you're alive now because (in most cases) your dad stuck his throbbing, erect penis into your mom's wet, quivering vagina once upon a time - and then you squeezed out of it!) ought to cater to them, to preserve their modest, easily offended sensibilities. Instead of, you know, acknowledging that sex happens, and is popular, and that its existence as a fundamental part of life is, I don't know, not a big deal, and that even though most people don't want to see people fornicating in the streets...if, for example, Janet Jackson accidentally exposes a nipple during a live televised broadcast, we should maybe, um, NOT get all bent out of shape and ohmigodshehasbreasts! and act like the world is gone to hell.
Meanwhile, we ought to begin to treat "perverts" as human beings, and not pretend like the fact that somebody likes to indulge in sex means they are not fit to join the family for Thanksgiving dinner, or that somebody likes to post sexy pictures of himself on the internet means they are not fit to make that presentation to the corporate office, or that somebody used to have a job as a stripper means they aren't fit to teach children in schools, for fuck's sake. Of course, there's that whole Moral Supremacy thing going on where sex is a vile sin, and immodest, unchaste people are devil's spawn, and the last thing we want to teach our next generation is that sex is natural and can even be kind of fun. Because, you know, teaching them that their natural impulse to procreate is the devil's temptation is totally healthy and everything. That's why modern, Western society (you know, the one that's shoving its ideals down the throats of indigenous cultures the world over - and then demanding they swallow) has such a healthy approach to sex, right?