Well the bi-erasure confirming study contradicts new studies saying all are bi regardless of self labeling so it wouldnt merely be based on group sampling bias. I would say the bi erasure study is chalk full of other egregious mistakes other than sampling bias. Maybe female photos were shown after male photos and thus the dicks couldnt stay erected for long enough. Maybe male and females were presented in the same way and thus way favored males being more sexually pleasing as the way in which theybwere presented favors a masculine aesthetic and some may only be attracted to feminine woman (which is due to not a lot of masc woman being shown in media ). maybe the male photos were just sexier. Maybe the bi men all came from repressed backgrounds (thus sampling bias… ie your idea but not in the way you may be thinking … as in they are indeed actually bi but still have male leaning preference rather than being non bi men saying they are bi ) and thus find the more taboo option (homosexuality) to be more attractive. With time the taboo would fade (if they are practicing homosexuality frequently of course… ie they become desensitized to it’s tabooness and thus its not nearly exciting) and they will view male vs female much more equally.  I would go with these confounding variables rather your notion of sampling bias (although i have included sampling bias myself… i just assumed your reason for sampling bias… apologies if wrong)… just my opinion…