One of the biggest problems with this sort of thinking is that it totally ignores that there was no such category as "homosexual" or "heterosexual" during the time Paul was writing—these are entirely modern ideas, with a provenance no earlier than the mid-nineteenth century. But, more insidiously, how does this then treat the phenomenon of bisexuality? What are bisexuals, other than people who (from the viewpoint of the straight world) have exchanged the "natural" for the "unnatural," and likewise from the gay and lesbian viewpoint aren't living up to what they "really" should be? I think this passage and its gay-and-lesbian-positive interpretation by certain gay and lesbian theologians has in fact done more to marginalize and erase bisexual Christians from the map than anything that homophobic theologians have tried to do.
And yet further still, the idea found amongst many bisexual activists, that all humans are "really bisexual" and just get socialized in one direction or another, is equally limiting and equally as triumphalist. There are people I know who have no revulsion nor ill feelings of any sort towards queer people, but who simply aren't attracted to their own gender; and likewise, there are gay and lesbian people I know who have no interest in people of genders other than their own. And, I think that's a wonderful thing, and a perfectly good thing, and should be "allowed" not out of courtesy or concession, but simply because it is a thing which exists.
I am not, by any means, against the usage of our very precise and descriptive language to attempt an understanding of our fellow human beings. I am, however, against the usage of such labels in essentialist and definitive manners in order to bring about a classification system of all humans, in which movements between or variations upon the classifications are not allowable. I am against triumphalism of every sort, whether it is homophobic and suggests that only straight people are worthy of equal treatment under the law and in religion, or that strictly gay people are in some sense "more spiritual" than their straight counterparts, or that everyone is "really" bisexual.
And, just as I think there should be more freedom allowable within gender and sexual expressions, I also think that there should be a consequent freedom allowed in terms of religious affiliation and orientation and (perhaps most importantly) exploration. Just because one flirts with polytheism shouldn't necessitate that one thus identify as a polytheist and nothing but; and the same goes with every possible religion to which one can be attracted or can potentially begin practicing. The same people who often wish to support an agenda of "heterosexuality only" are often the same people who think that their particular religious viewpoint and theology is the only valid one and the only one worthy of having full legal rights and recognition, even in a society like that of the U.S. in which all religious expression is supposed to be protected by law.
While I have great criticisms for many religious viewpoints, I am not under any illusion that my own religion, nor other forms of polytheism, are the "best" or the "most appropriate" for all humans. I have seen many people in many different religions live what I consider to be admirable and virtuous lives, both in the widest evaluations possible as well as under the conventions of their own religions. I do not wish to see this in any way curtailed in the future. Just as I stand against any notions of triumphalism in sexual orientation, so too do I stand against any such notions of triumphalism within religious viewpoints. Indeed, it is religious triumphalism which has caused a great deal of the difficulties of the last 1600+ years in the Western world, and as a result it is something which I do not wish to replicate in my own religious outlook, when I have the choice to prefer better alternatives.