Sexual orientations are probably the dumbest way throughout all of history to define sexuality and all the attempts to salvage the idea by making more of them have only called attention to how they instantly fall apart the minute you poke them with a challenge. Sexuality is fluid and nuanced; orientations are fixed and banal. I'd compare it to insisting that all of our sentences be forced to conform to strict rules of grammar, pounded into ""proper"" shape, rather than acknowledging that communication never needed or followed these forms in any strict sense.
Giving this question a satisfying answer demands that we throw this garbage framing device in the trash where it belongs. The actual common thread of masculine sexuality, one that can be found running through just about every culture, and what this question is actually asking is this: is it unmasculine to fantasize about being penetrated? The classical answer is yes. Pick a culture, any culture - there will be a history behind this idea, rooted in the dynamics of both war and peace, and practices will have reflected it. Whether it's men conquering and enslaving each other during war - an act consummated by sodomy - or an older man courting a young and beautiful athlete, the dynamics paint a picture of the penetrator keeping or even gaining masculinity and the penetrated losing it to some extent. While that's not the be-all-end-all (or even the prevailing) way we think of the subject today, that idea is far from dead.
But if we're talking about futas that's not -exactly- what we're asking about here. The whole concept of a futa presents the idea of a woman who is not confined to these classical social (or even the biological) dynamics - that is to say she can take both the social and sexual role of either sex or transcend both. Part of what we're asking, then, is "does putting the woman in the man's role necessarily mean that the man must be emasculated?". Now or then, that's not as clear-cut. The lion's share of femdom certainly takes that approach, but it doesn't -have- to; there's gentle femdom and even femdom that leans on mushy romance to the point where the label is more incidental than pathological - in other words 'respectful and lovey-dovey in tone, only female-dominant in the sense that she's setting the pace'. What we're brushing against this time is the old notion that a woman cannot respect a man - and a man cannot respect himself - if he's not completely in control.
A futa acting more or less masculine, taking the masculine sexual role, and emasculating the man during it will be insisting that anyone getting off to it couldn't possibly be manly. On the other hand, a futa acting feminine in the masculine sexual role, reinforcing and showing attraction to the guy's masculinity even though he's on bottom, would be sending an entirely different message. And it doesn't need to follow the strictest view of the roles as an either/or; a masculine-acting futa could be entirely respectful of the man's masculinity or a feminine-acting futa could infantilize the man by doting on him to the point where he comes off as boyish as opposed to manly or a sissy-boy altogether. Androgynous forms are special precisely because they can mix and match these frequencies or even rewrite them entirely. When we turn the aim away from pointless and reductive categorical word games and toward the actual psychosexual undercurrents an answer can emerge. It won't be the same answer for everyone, whether someone can deconstruct those associations between masculinity and sexual dominance will depend entirely on their own mindset, but finding it requires that we look at the source of the attraction.