I’m with @marinelife but for the aspect of women, as stated. Women’s fear about lack of attractiveness in men’s eyes plays less of a part…it’s not a cause so much as an effect.
Sex between women is acceptable if it is for men’s enjoyment. Because men have sexual agency, sexual agency for women upsets the patriarchal order. Therefore, the concept of them having sex for their own pleasure is unacceptable, as it suggests strength. Being gay, therefore, is upsetting regardless (although for men may be more ridiculed, but that doesn’t mean more acceptable). You would see much the same reaction when two “manly” or butch, or not stereotypically attractive women, were together, as you would with men.
Literally, you can trace the reason why these fears exist to the most basic method of control: property. Property can be retained only by exercising power over it. Without a government, this will most likely be done through the family. The larger the family, the more likely it is to be able to defend a larger amount of property, and the more able it is to accumulate resources. With a government, the people have either tacitly or expressly agreed to cede the means of control (violence) to a group outside themselves. The communicable expression of this agreement is law. Control of property (or power) is dictated by ownership rights. If men are the ones with ownership, they have power. Considering that humans, compared to every other species on earth (as far as I know), have to expend more resources and more time to bear and raise children given their development and size, women are at a disadvantage in being able to provide for children because they must physically take over these duties to some necessary extent. Therefore, men are the ones who naturally accumulate the resources to provide for the family. This is the reason why they may generally be provided with ownership under the law. In order to consolidate power in the family, property has to be transferable from one generation of the family to the next. This is where primogeniture (the passage of property held by the father to the first born son) comes into play…property is power, and if the family splits that between heirs (children) after the death of the owner (father) there is no consolidation, and each heir is weaker than the whole unit was previously. Therefore, men with only women children are more likely to cede power to another family through marriage because the power is consolidated and they can be cared for in their old age. And the only way that men can ensure that it is their family that can keep hold of the power is to control sexual access to women. Paternity has only recently become something that can be readily determined, but it is very, very clear what the maternal line of a child is (no doubt who your mother is). If women are allowed sexual agency prior to marriage, and are potentially not virgins at marriage, then it is uncertain that the man will be the father of the first son. If a woman has a child prior to marriage, it will be a burden on the resources of the future husband, and she will be less desirable, and therefore the father of that daughter will be unable to marry her off and will not be able to protect or keep property with age. This need for control continues through marriage because the husband needs to keep certain that he is the only one with sexual access to his wife to ensure that his family, and property, line can continue. Therefore, men are the ones who have the historical need to control sexual agency, and women are passive recipients (objects) of that agency.
Anything that is male, then, that is usurped by a woman, is threatening to that order. This has been steadily translated into a seemingly natural societal separation of the sexes. Women who act like men (with jobs, pants, and agency) threaten the consolidation of power in men. This is threatening, but if viewed as done to turn men on, plays into the roles above. Additionally, there is no chance that sexual contact between women will result in progeny that will be a future burden to the family of the woman. Men who act like women is disdainful because they are both giving up their power (rejecting their DUTY) to continue the consolidation of power and therefore will lead to the redistribution of wealth. It also demonstrates to society as a whole that such a man can be sexually objectified as well as be the sexual subject (the one possessing agency). If men can be sexually objectified, then it naturally follows in the heterosexual context that women could be the ones objectifying them, which requires that society recognize their sexual agency. The fact that men would both reject their duty to family (power) AND that they would threaten the power of all men around them CANNOT be absorbed into the normal power dynamic, and therefore is ONLY a threat to the patriarchy (as opposed to sexual contact (but not true lesbianism) among women). This also shows why those who do not conform to the heterosexual order are demonized, particularly with regards to the sexual danger to children (children are objects as well, which makes it oddly understandable that a man with agency could use a child, and also plays into the idea that male homosexuality is a threat to property as an exploited male child represents a theft of that family’s ability to consolidate power).
This is where these fears come from.