The issue is related to patriarchal views of male dominance.
Gender equality hasn't always been as prevalent as it is, especially in western culture. Women, from a cultural perspective, have often been viewed as "lesser than" their male counterparts. Thus, it is often an insult to a man to be feminized in any way. This is evident in derogatory language such as "pussy" to describe a weak man, or in sayings such as "

ou throw like a girl".
From a heterosexual point of view, male homosexual sex must force one or both participants into being "feminized", and essentially treated as a woman. This lowers their social status by seemingly taken on the role of what is perceived as the weaker gender.
This is ultimately what grosses most straight men out about homosexual sex. The notion that a man would freely allow himself to be feminized is socially repulsive to the average man.
In contrast, two lesbians engaging in sex are either feminizing each other - which is a status quo change of social class - or one is even asserting a dominate role, which is actually a climb up the social ladder. Therefore the act is not seen with nearly the same level as disgust as the same act between two men.
An interesting illustrative example is the socially condoned homosexual sex that existed in ancient Greek culture. Homosexual acts were widely accepted at the time; however, only between older men and very young men. It could only be that an older, bearded, man could have a relationship with a very young man, or even pubescent boy. Culturally, this was accepted, as the old man was feminizing someone of socially lesser rank, and therefore was retaining his dominant nature. Same-sex relationships involved two older men were seen as perverse and degrading, as at least one of the men would have had to allow himself to be feminized for the act to have taken place.
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