Why's that? Lesbianism/WLW tends to be more normalized and fetishized by men and society, and women aren't generally considered less feminine for being bi. The other way around though often makes people see the guy as less masculine. Its less normalized and people are more likely to just say the guy is gay and if he's a woman there's something hell always be missing.

No, they're right. People are much more likely to be jealous/skeptical/dubious of a same-sex metamour/ex/fling than opposite.