People get envious when they see a picture of someone much sexier that they ((possibly incorrectly) think they) are? I had thought of that... as a joke, but it hadn't occurred to me to take that seriously. (Wait, why does my brain think that what's funny cannot be plausible? It must be that, since if an idea is neither funny nor plausible I forget it shortly after hearing/thinking it, within the pool of ideas I do remember, being funny does negatively correlate with being plausible due to Berkson's paradox. Or something like that.) I'm thinking of how to test for this. (If this were right, women who think are ugly would object to such pictures more often than those who don't; also, objecting to such pictures wouldn't correlate much with religiosity, unless for some reason religious people are more likely to think they're ugly. Neither of these seems to be the case IME, but the sample size is small, I cannot always be sure whether someone thinks they're ugly, etc.) I do have a feeling that if I thought I was much uglier than I actually think I am, seeing pictures of half-naked sexy men would bother me much more, but I'm very bad at guessing what my feelings would be in counterfactual situations. (Hey, I do know a version of me with something like body dysmorphia -- that's myself from two years ago! Unfortunately, I can't remember any specific instance of seeing such a picture back then, and also I have changed in lots of other ways too so even if I could there would still be huge confounders.)

Another hypothesis is that one version is more offensive than the gender-reversed version because it's more common. Maybe I'm not bothered by pictures of sexy men because I don't see them that often, but I would get fed of them if I saw them several times a day; and maybe certain women are annoyed by pictures of sexy women because they see them all the time, but they wouldn't be if they only saw them a couple times a month.

Edit: OTOH, “just because you are right doesn't mean I am wrong”, i.e. it could still be that each of several causes plays a substantial role. What I've observed so far seems compatible with a model where that indignation is caused by:

1. a cached thought that erotica is undignified, originating from earlier, pruder times, most prevalent among religious/traditionalist/low-Openness people because that's the kind of people who hold onto cached thoughts from long ago; ISTM that this affects pictures of females more often than pictures of males (but I might be wrong about that). Often played for laughs;
2. people who think they are ugly getting envious when they see a picture of someone much sexier than they think they are. According to you it's more common among females, which seems plausible enough to me (though it's not like males talk to me that often about whether or not they think they're sexy, so I dunno); and
3. annoyance of people seeing something they're not interested in (e.g. sexy pictures of females, in the case of straight females or gay males) popping onto their news feed over and over again. Also happens with other stuff, e.g. football or gossip about celebrities.