There's a lot of pots to draw from here - homophobia within society (keep in mind I can really only speak for where I am in the US, since cultures variate here and there from country to country on how they look at it, and frankly the way the US is right now the culture in say, California or New York is vastly different from Texas or Arkansas or something), the way the patriarchy affects our view of all men and how that crosses over with how society views men who like men vs men who fit the heterosexual mold, sexism and how THAT affects our perceptions of others, sex and sexual liberation and how those things have historically tied to LBGT movements and the drawbacks as well as gains from those ties, personality traits leading to people making assumptions on what people are okay with, and probably a whole pile of other things I don't have enough knowledge of to even try and speak on.
Sexualization isn't inherently flawed, but when it overtakes a person's perception of something it is. EX: You can find m/m or f/f smut fiction hot without necessarily being a part of those communities, but it becomes a problem when you start attaching that sexual energy (? Er, words, I'm having a hard time with them today) to the m/m / f/f aspect and not the situation aspect (aka, the "they are having sex" part), and it becomes especially a problem when you start associating just m/m or f/f attraction with only sex/getting off, aka fetishizing. There's also the matter of humans often finding the "exotic" more interesting than what we know - I think that some of that interest, both for teen girls who like m/m and for teen boys who like f/f, does have a root in just...Being different than what experiences you could ever have. This is sort of putting aside the matter that sometimes looking for this can actually lead a person into explorations of the self - like, say, a teen "girl" really enjoying m/m stories, then slowly realizing they like it because well, they want to be in that...Specifically as a boy, specifically, they are a boy. Or a boy realizing they like guys too as they read more/seek out m/m as well as m/f and f/f...Which is a thing that does actually...Happen sometimes, as odd as it sounds. It's a really weird web to get wrapped up in, how materials that technically have a base in objectification+voyeurism (like say, BL manga, or fanfiction, or well, most romance novels/movies/anything tbh) can lead people to personal revelations, and how these things can both do positive things as well as negative things...People's perceptions variate so widely it's hard to say anything particularly concrete, imo. Two people can read the same thing and get wildly different understandings from it, thanks to a combination of peer group+personality+life experiences. I am very firmly rooted in "fiction is NOT reality", so I'm trying to be careful with how I put all of this. A boy can read+enjoy+get off to f/f fiction and not sexualize all female attracted girls in reality, same for a girl reading m/m...But that's an entire other thing.
There's the matter that men in general are often seen as sexual by default - not in the way women are seen as sexual objects, per say, but as participators IN sex. The reason you don't see as much sex "headcanons" for m/f couples I largely feel is related to this, because the thought process seems to be "guys like sex, therefore 2 guys = must REALLY be into it". And it's not seen as intrusive because they're two dudes, whereas with a woman/women it's like a mixture of how society likes to suppress female sexuality and how people are trained to see women as much more demure about their sex lives, how it's often insulting/degrading to bring up a lady's sex business, whereas with guys it's often more like a badge of honor, congrats you got laid. (Whether it is or isn't depends on the person I feel, I know some people who are very open about their sex lives with anyone, and some who keep that under lock and key.) That also ties with how Dan is more open about things in general than Phil tends to be, which...Leads to him being more objectified - people see Dan's openness with certain subjects (his kink jokes, the furry "jokes"we're onto you man, his general blase` attitude towards sexual references ("I love putting shells up my ass", ahem) ), and make an assumption from there than Dan is OK with people speculating about his sex life, and Phil just kind of gets dragged along for the ride since well, they're Dan AND Phil, after all. At least, that's how I see it.
There's also the ripple effect of fandom and what parts you go into - a fandom is a large group that's often splintered depending on ships. Even the phandom is splintered on the basis of whether you ship dan and phil together, or if you don't, or if you think they're together but you don't "ship" them with fanfiction and the like, or if you don't think they're together AND you don't do the ship thing, or if you think they're together AND you ship them. It's an even muddier thing because Dan and Phil are real people as opposed to fictional characters who can be whoever you want them to be, really, without any averse effect to them - Dan and Phil can actually see what people do and what people say about them. You can't actually control Dan and Phil the way you can characters that don't exist. If you come from a fictional fandom to Dan and Phil, you can miss how the game changes because Dan and Phil are real, actual humans, and for a lot of people that makes shipping them weird, which I get as someone in that general camp. It can feel intrusive based on your own personal preferences - I don't ship them because I wouldn't like that being done to me and my friends, but I understand that some people really would not even care. DNP don't even seem to care, they NEVER really seemed to care, it's only when people started trying to control them (stalking them, demanding they get together, harassing their families) that it became an issue for them. It became like a fan hassling a creator demanding their ship be canon, except these aren't fictional people, these are their Real Lives. (Not that hassling a creator to make your ship canon is ever good. Please don't misunderstand me there.) It was people taking their fanon and somehow convincing themselves it was reality that was the problem...That's always the problem, and I really don't know how to stop people from doing that because it tends to come with a certain type of fanaticism that comes with loving something a lot. I don't want to tell people "stop being so enthusiastic", but also...It might be good to take it down a couple notches before you trip and end up in a hole of fetishization and objectification. But that's just me.
IDK sorry for how long and rambly this is. I'm not sure a lot of it even makes a lot of sense. Hopefully I didn't piss anyone off.
and as mentioned, society does not bat an eye if 2 females are affectionate towards each other physically and emotionally. my sexuality was never questioned. 
I think I finally understand what people mean when they talk about fetishizing m/m relationships.
