It all comes down to a fear of being Other-ed.
Gender roles for straight men are extraordinarily strict. Look at mens clothing. How many colors are there to choose from? How many cuts? How much effort do straight guys put into not looking effeminate or queer or different? Tons. And all that effort stresses the straight guys out.
Straight Guys More Depressed and Stressed Than Out Gay and Bi Guys | Slog
While some folks—cough, cough—do sometimes question the professed sexual identities of some individual bi-identified guys, no one ever thinks an out bisexual guy is secretly straight. And no one scrutinizes out gay dudes for signs that they might not really be gay. Gay guys can be drag queens and/or leather guys (or both over one long weekend), we can be twinks or bears (or both at different stages in our lives), we can be musical-theater queens or heavy-metal fans (or both—wait, no, not both, never both). So long as you're into dudes and out about it, no one wonders if you might be secretly straight. A guy who comes out as queer is consequently free to do pretty much whatever and whoever he wants. He can be as butch or femme as he wants to be. Are you interested in stereotypically gay things like fashion, art, and musicals? Go for it, cocksucker. Are you interested in stereotypically straight things like football, baseball, and basketball? Go for it, cocksucker. You're free.
Straight guys aren't nearly as free—sexually speaking, I mean. Yes, yes: Straight guys run the world, the bastards, but when it comes to sexual identity, straight guys are pretty screwed. And this is because male straightness is defined by two negatives: To be a straight guy means (1) not being a girl and (2) not being a fag.
I don't know any gay guys who worry that people don't think they're gay. But I get letters every day from straight guys who are not only worried that people might not think they're straight, they themselves are worried that they're not straight. While gay and bi guys are free to do whatever girly or faggy shit we wanna do (look at the picture they used for story in the Telegraph!), straight guys are not allowed to do girly or faggy shit. And when a straight guy does something girly or faggy—or gets caught doing something girly or faggy—his sexuality is called into question. Did you have a feeling? You must be a fag. Did you attend a Broadway show? You must be a fag. Did you use a butt plug and like it? You must be a fag.
This isn't just something that straight guys do to themselves and each other: My Savage Love inbox overflows with letters from girls convinced their boyfriends are gay because their boyfriends like having their nipples played with or their boyfriends cross-dress or their boyfriends enjoy anal play. (I get letters every day from straight girls who are worried that their boyfriends are gay because their boyfriends want to fuck them in the ass!) It doesn't matter how crazy their boyfriends are about them or how much time their boyfriends spend facedown in their pussies. Just one girly and/or faggy interest—even an interest in anal sex—is enough to call a guy's heterosexuality into doubt.