It's mostly the odd distinction of "real" or "normal" men. Implying that gender-nonconforming/more feminine men are somehow faking being men or are less of a man because of their bodies. You're also catching trans men when you're saying this - a lot of trans men have 'giant tits' and/or a 'pronounced ass' and they're still men. Some may even choose not to get top surgery, and that won't change. Also cis men with gynecomastia, though they're obviously a completely different case.
There is also something I think is important to keep in mind here: this is a porn game. A porn game where exaggerated bodies and body parts, playing with gender and gender presentation, and femininity in general is a core part of the fetishes here. And thats also why it has a large appeal to a LGBT audience - the lines between genders are blurred, and you get to play in that in-between space. It's a space where you can have silicone tits and an ass the size of your head, but still be seen as a guy - or have that questioned in a fun sexy way.
Basically, a man can be a lot of things. A man can be even MORE things in a openly fetishy queer space. You don't have to get it- you just gotta respect it, yknow?