Many monosexuals think that their sexual orientation is given by nature and unchangeable. This idea got recent support from scientists who found differences in the brains of androsexual and gynosexual man. But, as for many scientific observations, many explanations are possible. It could of course be that humans are born with either andro- or gynosexual brains and that this determines their sexual orientation. But it is for instance just as well possible that a different lifestyle and different psychological problems that may come with a different sexual orientation give rise to a different development of certain brain fragments. For the time being I therefore believe that there is no scientific evidence for the naturalness of monosexual behaviour.
On the other hand there are various arguments against the naturalness of monosexuality. Pure monosexual behaviour among animals for instance is a rare phenomenon, that occurs mainly with species where the male is attracted by a smell that is emitted by females only.
But for me the most important reason to doubt that monosexuality is natural, is that I can hardly imagine how a monosexual orientation could be possible. Suppose that somebody claims to feel sexual attraction only for people whose length, rounded on entire centimeters, is an even integer. Would you believe that this is an inborn property? My argument against is that in order to determine whether somebodies length is an odd or an even number of centimeters, a precise measurement is necessary, and the (analytic) part of the personality that conducts the measurement has not much influence on the more unconscious part that determines if the measured person is sexually attractive.
It is a similar kind of reasoning that causes me to doubt the naturalness of monosexuality. Many people claim that they feel attracted to others mainly because of their mental properties. However, although there are differences between the mental properties of the average man and the average women, the differences among individual man and among individual woman are much larger. It could therefore be that if one is turned on by a set of properties that is characteristic for males, a female could occasionally appear that exactly has the desired properties. Mental characteristics are therefore insufficient to explain monosexuality.
In the category of physical properties it is sometimes argued that sexual contacts between man, or between woman, are less satisfactory due to a shortage of penises or vaginas. However, those who tried out such contacts and compared them with male-female contacts, tend to disagree with this assumption. Moreover, there are many forms of physical contact, like kissing and cuddling, that have not much to do with sexual organs but still suffer under monosexual behaviour.
Thus, if there is any ground for monosexuality other then our culture (i.e. prejudices and indoctrination), it must be the general shape of males and females. Androsexuals are turned on by male shapes and gynosexuals are turned on by female shapes. Although theoretically possible, I wonder if the differences in layout are truly sufficient to explain monosexuality. There exists for instance extraordinary ugly man and beautiful woman (and vice versa), even in the eyes of an androsexual. Would the androsexual then still prefer the ugly man above the beautiful woman? It is possible, but I find it difficult to imagine.
An illustration of my doubt in this matter is provided by a cartoon in a newspaper from the times that long hair for man started to get popular. On the first picture a man watches a long-haired person pass, and immediately little harts raise from him. On the second picture the man approaches the long-haired person, who on the third picture turns out to be male, and the man turns back ashamed and disappointed. This cartoon, that in my opinion displays our culture pattern very well (although the criticism on long hair has subdued in the meantime), indicates that monosexuality, and especially heterosexuality, comes with our culture and not by nature. The same can be concluded from this joke, that was recently sent to me by a presumably gynosexual male friend.
A strong argument for the thesis that monosexuality is a matter of prejudices, is the satisfaction of monosexuals with their sexual orientation. Imagine that one is born with a sexual orientation that limits sexual interest to people that are born on February 29, have a broken eye-tooth, and are with an accuracy of one centimeter 1.83 meter long. This would drastically reduce the choice of partners, making that one can not be too critical on the other characteristics of a potential partner. Moreover it prevents one from getting involved with somebody with many beautiful properties who happens not to be born on February 29. In such a case it seems rational to me to admit that one would be better served by a less restrictive sexual orientation.
Well, for the very same reason monosexuality is a deficiency, irrespective of the question whether it is an inborn or a nurtured deficiency. I would therefore expect confessions from heterosexuals in the spirit of ``OK, I'm straight, but I would prefer to be bisexual'', or at least ``I would equally much like to be bisexual''. However, most heterosexuals are happy with their handicap, and would find it particularly disquieting if they suddenly turned out to be bisexual. Of course this is as irrational as the thought ``happily I don't like strawberries'', or ``happily I am unable to enjoy art, nature or the company of fellow human beings''. My suspicion is that many monosexuals that would prefer to be bi, wouldn't stay monosexual for long. And those that are happy with their unilateral orientation are apparently indoctrinated by our culture and hence constitute no evidence for the naturalness of monosexuality.
On the other hand, there appear to be cases of homosexuals who would prefer to be hetero- (or bi)sexual, and in a phase of their live even believed they were heterosexual, but would never feel attracted to somebody of the opposite sex. Although several psychological explanations are possible, this could be taken as evidence that sexual orientation is a naturally given property after all. Whether of not this is the case is actually not all that important, but my amazement at the entire phenomenon of monosexuality remains.
By the way, the alternative for monosexuality is of course not bisexuality, a two-sided sexual orientation. I, for instance, am septisexual: I have a seven-sided sexual orientation. Namely, I do not have a dogmatic aversion from sexual contact with somebody who is born on a Monday, neither do I feel any repugnance to sex with somebody born on a Tuesday, and likewise for all days of the week.