Can anybody tell me why do gays is a synonym of being effeminate? but biual could be not so much? ?

Actually, my dear Honesty, I find "s" to be as often effeminate as gay men--perhaps even more effeminate. For one thing, biuals are most often not really s at all, but, rather, gay men in a state of denial. Their hypocrisy, not surprisingly, does not make them more masculine. Instead of anything like manly simplicity and candor, they have a vague, fussy, and fussed, mealy-mouthedness that seems feminine in all the bad senses of the term. Give me a straight-out nelly queen any day. If you're learning English as a second language, you'll find some really interesting usages in this answer.