The apparent paradox comes from the fact that sexuality for a lot of people is not a binary thing - gay or straight. There's a continuum, as Kinsey famously revealed. So the "straight" men who have recently had sex with other men might be 95% straight and just a little gay/bi - so maybe a 1 on the Kinsey scale. They identify with the label "straight", and it describes 95% of their attractions and behaviour so that's the label they use for themselves.
Then of course there is the "Latin Bisexual" phenomenon. Men who are "straight" but gladly let gay men suck their cock or bottom for them. They aren't attracted to men per se, they are more attracted to getting their rocks off and don't particularly care who is doing it for them.
Both of these help explain the results of the study.
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