According to Western philosophy (Christianity, Islam, Judaism) it is. The God in these religions says it is an abomination to lie with someone of the same sex as you would with someone of a different sex.
According to several religions, yes.
Some debate what God said, but two verses in Leviticus make it quite clear that sexual activity between two males is a sin. Bisexual behavior is not, however, clearly identified anywhere in scripture. Although, bisexual behavior demands sexual activity between one person and at least two others, and scriptural law is quite clear that adultery and fornication are clearly identified as sin, and one cannot engage in bisexual practices and not commit one or the other.
They can be, we all get the freedom to make our own decisions, being bisexual is never a sin.
Obviously the perspective of what is and is not a sin will depend on who you are talking to, as different Christians have diffenent views on sin and what the Bible commands. One Christian perspective would to say that having a certain sexual orientation is not a sin - but wrongly acting on that orientation - eg. to have sex outside of marriage or sex with a person of the same gender - would be.
The Church of Jesus Christ of Latter-day Saints (commonly called the "Mormon" church) teaches homosexual relationships are sinful. Being attracted to the same gender is not a sin, but following through on those attractions is. Therefore, being bisexual (admitting that you are attracted to both men and women) is not sin. Having a sexual relationship with someone that is the same gender as you (or anyone of the opposite gender that you are not legally married to) IS a sin. There are many gay and bisexual members of the Church, but they are supported in living a life of celibacy or marrying someone of the opposite gender, rather than being with someone of the same gender.You can learn more about what the Church teaches about homosexuality and bisexuality at the "Related Links" below.
It depends on what religion you're talking about. There are religions where it is considered a sin (particularly mainstream Christianity, including Catholicism and Mormonism), and religions were is is not considered a sin (most branches of Judaism, Unitarians, Pagans, some Christian denominations)
Sunflower is bisexual
Many believe that whether a person is gay, straight, bisexual, etc., He will judge you, no matter what, based on your life on Earth and how you did and didn't sin.
Bisexual as mango is.
Bisexual
No he is not bisexual.
When you are bisexual, you do the exact same things as a non-bisexual person.
She is definitely not bisexual.