"demifemme" is a gender identity for people who partially (hence the "demi-") identify as feminine (hence the "femme") and something else as well, instead of seeing oneself completely as a woman

"femboy" is nothing more than a kinky aesthetic for men who like to assume a more stereotypically "weaker" feminine role by dressing and acting effeminately in a primarily sexual context, or for men who are too shy to actually identify as feminine for the same reason of femininity being seen as a "weakness" that can damage their sense of masculinity so they playfully repress their femininity... a "feminine guy" like this is mostly just a mentally androgynous man to me

being a "feminine guy" or "femboy" is simply all about posing on the outside as what's commonly thought of as "feminine", like wearing girly clothes or trying to sound emotional and high-pitched, while being a feminine person means that you confidently express yourself like one from the inside and aren't preoccupied with fitting into the widely accepted image of girliness

sheldon is not just some effeminate cis guy, he's a genuinely feminine-expressing person: caring, gentle, sensitive, emotionally strong, passionate and confidently submissive, supportive rather than assertive... so more motherly than fatherly, he even imagined himself as a bad father figure once because he clearly wants to support his partner 24/7, not lead a whole family

my view of sheldon's gender identity is a whole can of worms in itself and my system of gender-related views would be mostly confusing or even shocking to others, so this is all i have to say on this topic