It's one of two things - awkwardness (in the case of discussing sex in our puritan-loving society) or privilege.
The first one is basically the weird and harmful relationship that a lot of our "civilized" societies have with sex. I want to say "Western" society, but it ain't the only one, and despite colonialism, I don't think they can really be held accountable for all of it (well... IDK maybe they can be - I only have spot knowledge of some cultures so I do not have the means to see a trend). Basically, having a HEALTHY relationship with sex is considered deviant. Unless people carefully examine their attitudes and realize that's a load of bull, they are naturally inspired to shield themselves from being perceived as deviant. Easiest way to do it is dismissal. It's unfortunate, but until a major societal overhaul, most people who grow up within the confines of that society will feel that natural inclination.
The SECOND one is a lot more pathetic, because it comes from a place of selfishness and juvenile lack of introspect. This one comes out when one person with very legitimate criticism points out something dumb that second person finds pleasing. For example, all female characters are pretty, young, scantly clad and second-class to the men, and all dudes are one step away from ogre and treated like gods. Woman points out that, hey, can we maybe not do this dumb thing? I also like to feel like a human being. Man responds with HUR FIKSHUN because he's secretly afraid that if someone hears her complaint, he will either have confront his closet bisexuality when the male characters become attractive, and/or he will have less material to jerk it too because of his lack of imagination and an inability to see women as human beings (or, without seeing constant reinforcement of the idea that he's superior just by virtue of being male, he's gonna have to confront his own mediocrity). Basically, it's the fear that IF the criticism is seen as legitimate, his interests will be catered to a little less, and hers a little more. It's pure selfishness at the expense of another person's humanity. Like, you can easily say "I hear you. It's weird that only the women are pretty and young. I like pretty people. I imagine you do too. Let's make them all pretty, because it's fiction, and we can do things that treat everyone's interests equally" or you can say "You know what, you're right. Seeing average/older men be successful and respected makes me feel better about myself and more empowered. I can totally understand why you want average/older women to be shown to be the same. Let's fix that, even if it means I'll have to dig a little harder (and maybe see women as a little more human) to find material to jerk off to". Both of those require perceiving another human without your privilege as someone being worthy of it. I used a man/woman example, but it applies to all forms of privilege. Straight/gay (fetishizing homosexual relationships for the sake of enjoyment by straight viewers), dominant race/non-dominant race (often times in the forms of diminishing the contributions and power of the non-dominant-race characters or by also fetishizing them)(this one is USUALLY white/non-white because... colonialism), cis/trans, etc.
