I waded into this in one of these other topics but one much more general thing to reiterate first: you're not going to understand too many of these sorts of things by approaching it from that whole "where does it fit on my gay and straight paradigm" since those are exceedingly narrow views that leave out...well, most of human sexuality. That's a huge and needless barrier to put over your viewpoint that'll conceal the appeal of a good many things even when they're relevant to them.
Now, there a number of ways to approach this one.
As mentioned specific traits are the basis of attraction so regardless of anything else finding those will trigger some. With American guys in particular the gay/straight thing is so emphasized that the confusion can actually be part of the appeal for them. It's a safe "dipping my toe in" way to experience a thrilling bit of taboo homo content without actually diverging from what they already know and like. Guys who're comfortable with their healthy attraction to or even a straight-out preference for masculine things find this far less appealing and in cultures where men are more comfortable bonding with and even exploring bits of light sexuality with other men this specific appeal is far less likely to titillate - but there's actually another way to approach this line, too. Deeply paternalistic societies where women are seen as filthy/dirty have a long, long history of this sort of thing. The Pashtuns of Afghanistan, for instance, have strict rules about how any sex with women is to be handled and women in general are seen as sinful so they developed a tradition where the older men would often take feminine-looking young boys as lovers (and these boys might often act like or girls or wear feminine clothes). When those boys grow up they might then marry a daughter of their former lover or otherwise take advantage of the connection and then take a boy lover of their own. And just as in American culture there's a strong insistence that this is different from homosexuality (since that's also sinful). So while the reasoning is slightly different this specific type of simulacrum/"safe gay" appeal is actually quite common in places where sexual repression and paternalistic worship of masculinity has shaped the cultural landscape.
That brings us to emasculation. There are branches to this one, too (cuckolding and "sissies" are probs the most well-known), but the central theme here is that the enormous pressure to be masculine in societies like these can make emasculation cathartic and very arousing. The erotic focus is either on some perceived failure to be masculine or on the more general idea of femininity as the absence of masculinity. With that idea in mind someone might crossdress to relive the constant tension of some high-pressure job where the person is constantly expected to be strong (masculine) or they could phrase it as having their manliness taken away as punishment for their so-called masculine failure. For guys who often feel like failures as men (and who have internalized this idea of femininity as lacking masculinity) taking this further to the point of being "turned into a woman" by crossdressing or letting a "real man" work over their wife (or even service someone who's seen as a masculine totem in their minds) or any other sort of thing that shreds their manly pride can be both comforting and hot for them. There's also some crossover with that whole "women are sinful/dirty" thing, so when they're playacting as a woman trying to be as slutty as possible while they worship masculinity is part of the experience of escaping it temporarily. There's a thrill to being the emasculator as a way of affirming your own masculinity, too, and once again this usually isn't seen as actual homosexual behavior since the thrill is more about making another guy their bitch than feeling any attraction to them.
Guys don't always subconsciously or blatantly view femininity as something dirty/slutty or as a simple lack of masculinity (hard as that is to get away from - usually it's more in terms of degrees or as part of a distaste for masculine things in general so its "lack of masculinity" is seen as a positive), though. Being feminine can be part of their general desire for femininity - women turn them on and so does looking and acting like one. Desiring and being desired, admiring beauty and wanting to be beautiful cross over to become the same desire. As with emasculation the role reversal can be arousing, too, though for different reasons. Most men feel constant temptation so being the tempter, the object of desire, the one who's beauty and approval is treasured for a change is another thrill and fits right into the desire to be seen as feminine and therefore pretty. They might do the whole "slutty girl" (or the cute girl or any number of other archetypes) act as part of this, too, knowing that a lot of men fetishize that and would find it appealing; it heightens the thrill of being lusted after and if they're submissive the ingrained idea of men = dominant and women = submissive can make them feel more feminine when they do those kinds of things. A lot of feminine stereotypes and even the realities of women's physical and social position can play right into a submissive's fantasies (the lack of strength/power/control) and enhance the experience for them. Even an attraction to masculine features can be enhanced when someone finds the masculine/feminine sexual dynamic to be the most attractive one; wanting to be and being treated as the "girl", even in mild ways, is a p. darn common thing that can get people's engines going for any or all of these reasons.
Shifting tracks, here's an appeal that doesn't get a lot of traction in Western societies but does have strong roots elsewhere: androgyny as a beauty ideal. In the Western view androgyny is often seen as a punchline or something slightly or blatantly unsettling but it's actually very common to view it as beautiful. A number of Middle Eastern nations, China, Japan, Thailand - in their histories things like makeup, soft features, long and silky hair, wearing perfumes and oils and so on weren't seen as strictly masculine or feminine but rather something for the wealthy and the pretty. Regardless of who was wearing the fashion the traits and the primping made the wearer look stunning and the appeal here is simple to understand: since these traits are seen as universally attractive men who would be called feminine-looking by the West are either regarded as gorgeous in general or even as macho in their own culture.
As long-winded as this ended up being that's just a handful of reasons. Personally the big appeal is a hermaphroditic "yin and yang" balance of masculinity and femininity where the two individual states are transcended and the feminine guy finds a state where he's both a girl and a boy at the same time, always with the potential to fill either role but never being restricted to one or the other. For me it hits this more than actual hermaphrodites do since they're rarely "in between" - usually they're blatantly one or the other.
