The likely reason for this difference in body image among men, depend primarily on how men and women perceive sexual attraction and respond to sexual stimuli.
Most women do not objectify men sexually or put great emphasis on sexual physical attraction as men.
However, men do objectify women sexually, as it is a crucial part of their male sexuality and there is no real choice here, because if men do not perceive women as ‘object of sexual desire’, then sexual attraction and desire to experience sexual intercourse will unlikely arise in men. Thus, there's nothing wrong in sexual objectification. We see plenty of evidence of this phenomenon in the modern world, all around us today from the subtle to the very obvious signs of sexual objectification.
Hence, in essence women do not objectify men sexually but women almost always will be “object of sexual desire for men. This is a just a fact of Life. I am not saying women cannot or will not objectify men, as some women may very well objectify men sexually but predominantly women's sexual attraction is quite subjective in nature and very context dependent.
However, for men it is almost a ‘must’ to sexually objectify women, and if any men say otherwise, highly likely they are being dishonest for some reason or subconsciously objectify women.
Sexual desire in men is spontaneous and men find visual sexual stimulus very arousing. There many studies that conform these findings. The reason is because in male sexual behaviour, erotic visual cues are purely related physically and psychologically to a normal sexual arousal, resulting in an instant penile erection.
Men also have somewhat swift and congruent subjective and genital sexual arousal.These findings are irrespective of a man's sexual orientation.
In fact, unlike women who can be aroused by the both sexes and have a very context dependent responsive sexual desire, men have very distinct sexual attraction based exclusively on the sex they are attracted, depending on the man's sexual orientation.
Therefore, men routinely only get sexually objectified by other men who desire them sexually, namely gay/bi men. This is a very common perception among almost all gay men. One just has to visit any gay bar, and will immediately understand how a man can be sexually objectified easily, a gay bar is like ‘hook up central for men’. As gay men are still men, so their psyche also works exactly same as straight men, except that, the type of sex they attracted to is inverse.
This is also the reason gay men have the highest number of sexual partners compared to all other sexual orientation demographics.
On the other hand, lesbians and heterosexual women have similar sexual nature. Hence, even if straight men desired to have sex as much as gay men, usually the possibility is compromised by women's unwillingness to do so.
Evidence of this male sexual nature is clearly seen in the porn industry that predominantly caters for the needs of both straight and gay men exclusively. This includes the similarity in the genre of porn type which very often emphasizes on sexual objectification irrespective men or women. So "boys will be boys, even if they like other boys".
This I think is the main and primary reason gay men have lesser positive feeling about body image compared to straight men, quite similar to women. except among homosexual men, they are both subject and object of sexual desire.
In the end, it all essentially just comes down to inherent sexual nature and natural sexual attraction.
In my view, it is less to do with appearance-potent subculture among gay men or the fact gay men are more marginalized to explain the negative body image perception. These can be contributing factors but not going the major reasons.