The assumption that gay men are either a top or a bottom is relatively new, historically speaking. I am old enough to know.
In the early 70s, the "Advocate" included the personals section with every issue. Guys looking for partners stated explicitly what they wanted. Oral was called French (abbreviated F), anal was called Greek (abbreviated G). Active was A, passive was P. The common designations were FA, FP, GA, GP, and V (for veristal). In scanning the personals, it was clear that GA and GP were much less common than FA and FP. There was also more flexibility, such as FA / FP.
The change in how gay men expect to have sex appears to have occurred in the late 70s and early 80s, after which anal sex became much more common and it even became assumed that anal was the ONLY way that gay men had sex. An unfortunate consequence of the change was (and remains) the AIDS epidemic. Anal sex is the most efficient means of transferring HIV from one partner to another; the risk is so high that it has been estimated that if the "top" is HIV+ and the "bottom" is HIV- and no protection is used, the probability that the "bottom" will become HIV+ may even exceed 50%!! Experts believe that HIV infection from oral sex is uncommon, although possible (although other STDs can easily be spread via oral sex). Anal sex is a much more efficient way to transmit HIV than vaginal sex.
The anti-AIDS establishment promulgates messages based on the assumption that the ONLY way gay men have sex is anal. Those messages probably influence young gay men to believe that they have to have anal sex. It would be better if the anti-AIDS establishment changed their messages to make it clear that there are less risky techniques for having sex.
Had it not been for the shift from oral sex to anal sex, the AIDS epidemic would be only a fraction of the problem that it is now. Attempts to change the way that gay men have sex are usually met with disbelief that change is possible. Those of us who don't want anything to do with anal sex are treated as eccentric and are often called immature or other names. There is definitely discrimination.
In my view, anal sex is so dangerous that, although condoms do considerably reduce the risk of HIV transmission, even a small risk of condom failure still results in an unacceptably high risk if someone who is HIV- often has sex with someone who is HIV+ or of unknown HIV status. In case of exposure to HIV, the likelihood of its taking hold can be greatly reduced by taking anti-retroviral medications for 30 days, provided that the therapy begins very quickly. So, in case of exposure, it would be a good idea to go onto medication within hours, which could be difficult to do. I have no idea what would happen if, because of condom failure, someone presented himself at an emergency room to get anti-retroviral medications.