It's helpful to look around the world and history, to see that ideas about what is masculine or feminine, is completely fluid. Almost nothing about our concepts of gender, are biologically based.
So looking at my own stereotypes, ideals, ideas: I'm masculine, attracted to stereotypical "rugged masculinity" in men. Respect femmeboys as people but am not attracted categorically, though for me attraction is more like two levels: noticing appreciatively (and occasionally trying to ogle surreptitiously), and then becoming intrigued with someone if I get to know them and there's something there, at which point I tend to like their features because they go with the person, so in that way, I could conceivably become attracted to a femmboy, they just don't "make me look". Certain women make me look, also.
I respect other people's right to express themselves even if they do it in ways that I personally wouldn't do or outright don't like, aesthetically. I personally don't want to express myself by wearing a dress, and didn't think Harry Styles looked super in one either, but then again, he's not what I would find attractive no matter what he wore. And nail polish. I don't think it's attractive on anyone and don't like it on myself either. But I wouldn't look down on someone else just because they like that, or to wear a dress if they're a guy. It's their choice, and no one's making me do it.
1: How do you define masculine?
Definitions are hard. Vague stereotypes are the best I can do. Masculinity positively correlates, vaguely, in my eyes, with strength and fortitude, self-control, confidence and assertiveness. Confidence in one's masculinity is easy to see, because those people aren't walking around with a chip on their shoulder, picking fights and taking offense at trifles. And confident, relaxed masculinity can show tenderness, kindness, empathy, and seeks a win-win, gives the other side a way to save face. Real leadership, the kind that can inspire loyalty in the opposition and have everyone part as friends, is the pinnacle of masculine virtue, but could also apply to women. Being mindlessly violent, aggressive, cruel? that's not masculine, that's just broken. Unfortunately the stereotype that men aren't supposed to have feelings or show tenderness, breaks a lot of boys' hearts and teaches them not to show it, and they grow up as broken men.
3: What career would your masculine sim like?
My masculine Sims are often musicians, reflecting my own interest. I also seem to play ones who have an ambition to be tinkerers/inventors, or some kind of Mad Scientist. :D
4: What would your masculine sim like to wear?
What my ordinary-joe masculine sims wear: cargo shorts/tropical shirts/short-sleeved plaid oxfords (or t-shirts if they are teens) in summer, and plaid buttondown over jeans, sometimes with a t shirt under, or hoodies, with boots, in cool or cold weather. Summer sleepwear, tighty whities (available in Sims3) otherwise boxer briefs (sims 4) and in cooler weather, usually drawstring pants, no shirt or sometimes a t-shirt depending. If it's a different type of character, that would have unusual clothing choices because of a strange storyline, like being a gothy vampire or something, then I dress them in character with that, but the above is my usual.
5: Does your masculine sim have/want body hair?
YES! Haha part of that is aspirational, my hair is pale and sparse, and I very much like body hair on men. Wouldn't dislike someone just for lack of it, but yeah. Masculine Sims in Sims4 who look like they ought to have body hair to me, but don't, look jarring, so I use a mod for Sims4 for body hair. Even hairless-chested Sims will usually have a landing strip and pit hair.
