Answer:
No, it is not. Unless you belong to a religion that says only straight people are sinless. I cannot change any religion's teachings -- so if you belong to one of those, and they say there is only one kind of sinless relationship then I guess the answer for that religion for gay and bisexual people alike would be 'yes'.
But the idea that bisexuality is a sin 'because' bi people are always having sex with both men and women is absolutely wrong. Bisexuality is a sexual orientation that means a person is capable of loving either men or women over the course of a lifetime... not at the same time. Bisexuals are as monogamous as straight or gay people (no more or less). The idea that the definition of bisexual means you are always in 2 or more relationships, one with a man and one with a woman, is a huge error.
So if, like the previous answer, you are asking if it is a "sin" because of the idea of multiple relationships, then again the answer is "no, bisexuality is therefore not a sin" as there is no need for a bi person to ever be in more than one relationship.
[Previous answer: If you ask me the answer is yes. As you are having sex with a male as well a female.]